An International Journal of Cosmogenic Isotope Research

Radiocarbon

VOLUME 42 / NUMBER 1 / 2000

SPECIAL ISSUE

In honor of Renee Kra, Managing Editor for nearly three decades

• Tributes and color photographs • Retrospective articles • The future of 14C and AMS, and more

Guest Editors E MARIAN SCOTT DOUGLAS D HARKNESS

Editor A J T JULL

Associate Editors J WARREN BECK GEORGE S BURR

Managing Editor KIMBERLEY TANNER ELLIOTT

Department of Geosciences The University of Arizona 4717 East Fort Lowell Road Tucson, Arizona 85712-1201 USA ISSN: 0033-8222 RADIOCARBON An International Journal of Cosmogenic Isotope Research

Editor: A J T JULL Associate Editors: J WARREN BECK and GEORGE S BURR Managing Editor: KIMBERLEY TANNER ELLIOTT Interns: JACKIE LIND and MARK MCCLURE Subscriptions and Sales: KASHO SANTA CRUZ Managing Editor Emerita: RENEE S KRA

Published by Department of Geosciences The University of Arizona

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RADIOCARBON is indexed and/or abstracted by the following sources: Anthropological Index; Anthropological Literature; Art and Archaeology Technical Abstracts; Bibliography and Index of Geology (GeoRef); British Archaeological Bibliography; Chemical Abstracts; Chemistry Citation Index; Current Advances in Ecological and Environmental Sciences; Current Contents (ISI); FRANCIS (Institut de l’Information Scientifique et Technique – CNRS); Geographical Abstracts; Geological Abstracts; Oceanographic Literature Review; Science Citation Index; Social Sciences Citation Index. RADIOCARBON, Vol 42, Nr 1, 2000, p vii–xvi © 2000 by the Arizona Board of Regents on behalf of the University of Arizona

A TRIBUTE TO RENEE KRA: RADIOCARBON MANAGING EDITOR FOR 30 YEARS

Renee and Doug Harkness (the cowgirl and the kiltie) sample Scotch whiskey near Loch Lomond, Scotland during a mixer at the 1994 LSC Conference.

An author begs Renee for a later submission deadline at the June 1988 14C Conference in Dubrovnik, Yugoslavia.

vii viii A Tribute to Renee Kra

Left: Renee is caught by surprise while dancing with Gordon Cook during the Second International Symposium on 14C and Archaeology in Groningen, the Netherlands, in September 1987. Right: Renee tours Helsinki in May 1990.

Left: Renee is joined by friends in June 1990 at the University of California Lake Arrowhead Conference, the proceedings of which were published jointly by Radiocarbon and Springer-Verlag in Radiocarbon After Four Decades. Former Editor Austin Long is seated to her left. Right: A clearly labeled Renee prepares to board a tour bus during the August 1979 14C Conference in Bern and Heidelberg, Germany (apologies for the photo quality). RADIOCARBON, Vol 42, Nr 1, 2000, p 1–21 © 2000 by the Arizona Board of Regents on behalf of the University of Arizona

THE CONTRIBUTION OF RADIOCARBON DATING TO NEW WORLD ARCHAEOLOGY

R E Taylor Radiocarbon Laboratory, Department of Anthropology, Institute of Geophysics and Planetary Physics, University of California, Riverside, California 92521 USA. Email: [email protected].

ABSTRACT. When introduced almost five decades ago, radiocarbon (14C) dating provided New World archaeologists with a common chronometric scale that transcended the countless site-specific and regional schemes that had been developed by four generations of field researchers employing a wide array of criteria for distinguishing relative chronological phases. A topic of long standing interest in New World studies where 14C values have played an especially critical role is the temporal framework for the initial peopling of the New World. Other important issues where 14C results have been of particular importance include the origins and development of New World agriculture and the determination of the relationship between the western and Mayan calendars. It has been suggested that the great success of 14C was an important factor in redirecting the focus of American archaeological scholarship in the 1960s from chronology building to theory building, led to a noticeable improvement in US archaeological field methods, and provided a major catalyst that moved American archaeologists increasingly to direct attention to analytical and statistical approaches in the manipulation and evaluation of archaeological data.

INTRODUCTION The aim of this discussion will be to summarize the most important contributions that 14C age determinations have made in understanding the process and pace of culture development of human societies in the Western Hemisphere. In reviewing these contributions, it might be helpful to note several conceptual and historical factors that condition how 14C values have been employed in New World archaeological studies in comparison and contrast to their utilization in other areas of the world. First of all, the entire period of human occupation of the Western Hemisphere involves the activities of anatomically modern Homo sapiens sapiens. With few exceptions, no competent researcher has proposed the existence of any pre-sapiens hominids in the New World. The consideration by paleo- anthropologists of the chronological problems and issues involving the geochronology of Pleistocene fossil hominids are exclusively the province of the students of Old World archaeology. Secondly, with one major exception, pre-Columbian New World societies did not possess textual-based records that survived for modern scholars to examine and thus there were no historical-based chronological systems to which archaeological features could be associated. The great exception is the textual tradition created by priestly elites of the Maya of the Yucatan Peninsula of ancient Mesoamerica (Mex- ico). Except for this textual corpus and the corpus of codicies that record pictographically events occurring in the few centuries before European contact in central Mexico, several other areas of ancient Mesoamerica and in a few other areas in the Americas, materials recovered through archaeological excavations provide the sole data base on which the reconstruction of the cultural history of the pre-European societies of North and South America can be based. In this sense, with the exceptions noted, New World archaeological studies have been undertaken within the intellectual contexts involved in the examination almost exclusively of nonhistoric or prehistoric societies. This has meant that, following the introduction of 14C dating, the most straightforward unit of New World archaeological chronology could have been expressed simply in “radiocarbon time” as in BP (before present), which will be followed in this discussion. This is in contrast to European practice, where chronology building included direct links to the historic chronologies of the circum Mediter- ranean and Near Eastern civilizations conditioned a need to convert 14C-based BP into calendar-based AD/BC units. It is true that New World archaeologists, in many cases, have followed the practice of converting BP to AD/BC units, perhaps influenced by the early editorial practice of Radiocarbon. This was necessary for the southwestern United States where comparisons with den-

1 2 R E Taylor drochronological data was required, and in eastern Mesoamerica where comparisons with Maya long count calendar based chronologies existed. However, in light of the subsequent calibration problems, in all other areas of the New World, it might have been prudent, from the beginning, to have represented their chronologies exclusively in radiocarbon time. Finally, as discussed in detail elsewhere (Taylor 2000a), an important feature of the development of professional prehistoric archaeological studies in the United States has been its 20th century development while almost entirely embedded within the American anthropological tradition. This characteristic is in significant contrast to the conceptual, intellectual, and organizational environment that developed autonomous prehistoric archaeological disciplinary traditions in England, other European nations, and academic traditions based on European models such as that which developed in Canada and South America. The final years of the 20th century mark the completion of five decades of the use of 14C dating in New World archaeology. As James Arnold, one of the two coworkers with Willard F Libby (1908– 1980) in the initial development of the 14C method, many years later remarked (Arnold 1992:3), the origin of 14C dating could be set as early as 1946—the date of the first paper on “radiocarbon” (Libby 1946)—or as late as 1951—the first published 14C date list (Arnold and Libby 1951). If an actual “birthday” for 14C dating is desired, it might be identified as the day on which the first 14C “date”—an Egyptian archaeological sample—was actually calculated. This was July 12, 1948 (J R Arnold and E C Anderson, personal communication 1996).

Radiocarbon Dating: An Archaeological “Atomic Bomb” An American archaeologist who quickly became associated with Libby in introducing the 14C method to his colleagues, the late Frederick Johnson (1904–1994), once remarked that 14C dating dropped the equivalent of an “atomic bomb” on archaeology in the late 1940s (Johnson 1965:762). His initial comments were focused on North American archaeology, but he also later applied it to the whole of the Western Hemisphere (MacNeish 1996). Johnson was in an excellent position to evaluate the initial impact of 14C dating on New World archaeology both to his peers within Americanist Archaeology and to archaeologists dealing with Old World issues and topics. He had served as the president of the Society for American Archaeol- ogy in 1946–1947 and, beginning in 1947, prepared an annual report on American archaeology for the American Journal of Archaeology. In 1950, Johnson reported that some of initial set of dates:

agree fairly well with established ideas, others indicate that archaeological guesses. . . while perhaps of the right order, relatively, are nowhere near the actual age . . . In some cases they seem to contradict what has been assumed to the “fact” based on stratigraphy, and in other cases they necessitate drastic revision of present conceptions (Johnson 1950:236). Throughout the 1950s, Johnson (1951, 1952, 1955) would provide the most authoritative and informed commentaries on the increasing corpus of 14C values and reflections on their increasingly important role in the derivation of temporal relationships in New World archaeological studies. He recorded the initial resistance of a number of American archaeologists to what was perceived in some quarters as a “threat of the atom in the form of radiocarbon dating” who then added that “this may be our last chance for old-fashioned, uncontrolled guessing” (Phillips et al. 1951:455). Johnson recalled the “frequent howls of protests, often savagely derogatory” (quoted in Marlowe 1999:22) as various 14C results diffused through the North American archaeological fraternity and reported that: Contribution of Radiocarbon Dating 3

Libby’s proposal to provide a new means of counting time, one which promised a definable degree of accuracy and world-wide consistency, caused all sorts of consternation. The idea that the method of dating was derived from nuclear mysteries . . . converted surprise to fright, and sometimes even panic (Johnson 1965:762). A summary of the 14C values determined at the University of Chicago laboratory during the period of its operation (Table 1) reveals that archaeological samples constituted more than 60% of the total. Of the archaeological samples, about 70% were from New World sites. It appears that the initial reaction of a number of New World archaeologists to the introduction of 14C dating was not that different from how it was initially received in certain quarters in European archaeology (e.g. Neustupny 1970). One difference appears to be that very little documentary evidence of the most hostile and derogatory comments from New World prehistoricans has survived—most of it being transmitted only orally in informal gatherings. Johnson reported that there was an unconfirmed rumor that a scientist who once proclaimed that “radiocarbon dating would never work” had later destroyed incriminating correspondence (F Johnson, personal communication 1986).

Table 1 Chicago 14C determinations: discipline and regiona Discipline Region Archaeology Geology Otherb Western Eurasia (Near East) 33 ——[9%] Western Europe 7 10 — [5%] England 4 14 — [5%] North America 128 78 21 [60%] Mesoamerica (Mexico) 21 5 — [6%] South America 18 4 5 [6%] Otherc 31 2 — [9%] 242 [63%] 113 [30%] 26 [7%] aTotal N = 381 (Libby 1955) bMaize, tree rings, guano cJapan, Sub-Saharan Africa, Hawaii, Australia

In the mid-1960s, Johnson (1965) commented that the impact of 14C on the archaeology of the West- ern Hemisphere was most significant in resolving the chronology dealing with antiquity of New World human populations, the age of the Adena and Hopewell cultures of the Ohio valley, the beginnings of New World agriculture, and the correlation of the Maya calendar with the Western calendar. He also noted that, in his view, the “inevitable period of readjustment following the initial hue and cry [when 14C dates were first introduced] brought about a reappraisal of the archaeological evidence” in a number of sites. In this connection, he commented that:

This return to the trenches for a new and more careful look, often for the purpose of proving the radiocarbon dates to be erroneous and useless, resulted in a refinement of methods of recording in the field to in order to determine more precisely associations of samples with levels. We may not realize it now, but possibly this need for detailed and accurate record of the provenience and associations of samples has resulted in material improvement of archaeological field methods (Johnson 1965:764). By the time of Johnson’s 1965 commentary, which appeared as an article in the first international radiocarbon conference proceedings volume (Chatters and Olson 1965), 14C dating can be said to have fully emerged from its pioneering phase. The period of initial suspicion and even, in some quar- 4 R E Taylor ters, hostility (F Johnson, personal communication 1986) which questioned the general, overall validity of the 14C method in toto were, with few exceptions, now silent and discussion turned to questions of the accuracy and precision of 14C values from specific archaeological or geological contexts. The journal Radiocarbon (originally the Radiocarbon Supplement to the American Journal of Science) was now in its seventh volume. By this time, more than 20 14C laboratories had been established in all parts of the world. In the United States, at three of these laboratories—at the Universities of Arizona, Michigan, and Pennsylvania—archaeologists were instrumental in their establishment.

CULTURAL SEQUENCE TO CHRONOMETRIC AGE The effects of the introduction of 14C data into North American archaeological studies dealing with the development of the various culture histories varied depending on several factors but among the most important were the traditions that had developed in different regions of North America among area specialists prior to the advent of 14C method. Johnson’s list of important issues that 14C data was of particular importance—the age of the Adena and Hopewell cultures of the Ohio valley, the beginnings of New World agriculture, and the correlation of the Maya calendar with the Western calendar—will be considered in this section. We will consider the influence of 14C dating in studies concerning the early peopling of the New World in a separate section since New World Paleoindian archaeology transcends regional considerations.

Eastern North America James Griffin, the archaeologist who had developed the first major pre-14C synthesis of Eastern US prehistory (Griffin 1946 [actually written 1937]), was directly involved as a central figure in the introduction and articulation of 14C data into the existing matrix of North American archaeology with particular focus on Eastern North American (Griffin 1952, 1967; Stoltman 1978). Griffin was also largely responsible for the development of the University of Michigan 14C laboratory, one of the pioneering 14C facilities in the United States which, throughout its more than 20-yr history (1950– 1972), focused its attention on archaeological samples. Just as 14C dating was being introduced, Griffin (1952), and Griffin together with Philip Phillips and James Ford (Phillips et al. 1951), had undertaken a review of the chronology for the ceramic periods of the Lower Mississippi Valley. While their analysis pushed back the Mississippian and early ceramic complexes of the region to close to the ages indicated by the first set of 14C values for the region, Griffin (1978:55) would later comment that 14C values:

altered many of the earlier interpretations of the temporal position of a large number of the cultural complexes recognized by archaeologists. Probably the most important changes have been within the time period attributed to the Archaic cultures, but all other periods have also been affected by the more accurate temporal assessments provided by radiocarbon. One of the most surprising results of 14C dating was the age of the fiber-tempered pottery in southeastern United States in the Savannah River area and in northeastern Florida. The earliest pottery appears about 4500 BP in Georgia and about 4000 BP in Florida. 14C dating contributed significantly to eliminate the possibility of a Eurasiatic origin for Eastern Woodland pottery (Griffin 1968). Much interest and initial controversy surrounded the dates associated with two complexes associated with burial mounds in the Ohio Valley and adjacent areas. The temporal relationship of Adena and Hopewell and how 14C data were to be interpreted in attempting to resolve their chronological relationship occupied the attention of archaeologists in the region for several decades. As was the case in several other regions (e.g. initial 14C dates dealing with Maya long-count calendar), initial Contribution of Radiocarbon Dating 5 age determinations prompted much consternation as the temporal priority of Adena was called into question. It took more than a decade for the difficulties to be at least partly resolved with the recognition of the lack of association of samples with their alleged archaeological contexts, distinctions between burial and village manifestations, and the temporal overlap between Adena and Hopewell in some areas. The advent of agriculture in the Eastern United States had been an important regional horizon marker since the absence or presence of agriculture had been a principal feature that initially served to differentiate the Archaic from the later Woodland and Mississippian periods. There had been a tendency for some to claim agriculture as an independent invention in the region, but none of these suggestions could stand as more and more 14C values modified views as to the “first” occurrence of different cultigens in the region. In the 1980s, AMS-based 14C values on carbonized fragments of squash (Cucurbita sp.) confirmed its occurrence in Archaic period deposits in Illinois at about 7000 BP. By contrast, AMS analysis of fragmentary remains of maize (Zea mays) previously dated at about 2000 14C yr on the basis of associated organics determined that the actual age of the maize itself was about 1500 BP at one site. In one case, the maize fragments were determined to be modern contamination (Conrad et al. 1984). However, later studies also using AMS technology to obtain direct dates on maize determined that it was present in Middle Woodland period about 2000 BP in the upper Mississippi River valley (Riley et al. 1994). Accelerator mass spectrometry (AMS)-based 14C determinations were also used to document conclusively the indigenous occurrence of a North American plant thought by some to have been introduced at the time of European contact. Individual seeds of Corispermum L. were analyzed to eliminate the problems of stratigraphic mixing and late Pleistocene/early Holocene ages were obtained on four specimens (Betancourt et al. 1984). Likewise, AMS-based analysis directly on samples of common beams (Phaseolus vulgaris) from sites in the Northeastern United States determined their occurrence in the region not earlier than about 650 BP (Hart and Scarry 1999) in contrast to earlier 14C values on purportedly associated charcoal (Richie 1969).

Western North America Like many areas of the New World, chronological understandings concerning the prehistoric cultures of Western North America can, with justification, be divided into “pre-14C” and “14C” eras. Although early excavations in Lovelock Cave, Nevada benefited from the introduction of stati- graphic strategies introduced by N C Nelson, until the late 1940s, with few exceptions, the majority of the archaeology conducted by those with various levels of training and experience focused on artifact collecting. Chronological relationships were initially structured by comparing Great Basin materials with those from other regions, most notably, the Southwest (Heizer and Hester 1978). However, a focal feature of traditional chronology building in the Great Basin has been the use of projectile points as time markers and much interest was focused at the time of the introduction of 14C—which continues to the present—on the relationship of various dated series—mostly in caves and rock shelters—with the various regional projectile point series (Hester 1973). This is of particular importance in a region where most sites are surface features and the projectile points themselves are the sole means of establishing temporal control. An important theme in Great Basin archaeology has been various views concerning various lengthy abandonment(s) of parts of the region. In some versions, this argument was associated with arguments concerning the date of the introduction of the bow and arrow to the region. Region-wide abandonment was first postulated to coincide with the Altithermal, a period of decreased effective mois- ture dated to between about 7000 and 4500 BP (Baumhoff and Heizer 1965). Other interpreters 6 R E Taylor suggested human abandonment of the entire eastern Great Basin between about 3200 and 1500 BP (Madsen and Berry 1975). A significant element in the arguments supporting such a view are interpretations of 14C dates and associated cultural materials in a series of caves and rock shelter contexts. A critique of this view notes that the use of 14C values for such purpose is usually very prob- lematical in part due to the differential availability of datable materials from closely spaced levels in sites and the high cost of obtaining sufficient 14C analyses to permit the secure documentation of any such hiatus context (Aikens 1976). Great Basin and California archaeology has yielded a number of case studies of the effect of the loss of stratigraphic association for samples in cave and rock shelters that can occur both during occupation as well as after the abandonment of sites. An example of such a problem is illustrated by the 14C analysis of materials from Gypsum Cave, Nevada. Atlatl shaft fragments and dung from an extinct giant ground sloth were found in apparent association. An early 14C date obtained by the Chicago laboratory on the dung determined it age to be approximately 10,000 BP. This value was used to infer an age for the atlatl fragments (Libby 1952). More than a decade later (Berger and Libby 1967), a 14C analysis directly obtained on one of the atlatl shafts determined that their age to be about 3000 BP. The same difficulty was encountered in determining the age of fragments of atlatl dart shafts that had been recovered from Potter Creek Cave in northern California in the early part of this century. The dart shaft fragments were originally thought to have been contemporaneous with extinct Pleis- tocene fauna found in the cave. The 14C age of the atlatl shaft fragments were later determined to be about 2000 BP indicating at least a 6000–8000-yr temporal hiatus between the extinct fauna and the cultural materials in this cave (Taylor 1975; Payen and Taylor 1977). For northwestern North America, 14C values provided the chronological definition for a major Holocene time marker used by archaeologists and other Quaternary scientists in the region. A vio- lent eruption of Mount Mazama in the southern Oregon Cascade range, one of the largest eruptions of the late Quaternary, distributed volcanic ash from central Nevada to British Columbia (Bacon 1983). The event has been characterized by mineralogical analysis of tephra sediments in many archaeological sites, geological contexts and lake sediment cores throughout the region. A recent review of the 14C determinations associated with the eruption have identified 65 values obtained by 16 laboratories over the last five decades (Hallet et al. 1997:Table 1). Critical reviews of this data place it within a century of 6800 BP with one investigator calculating 6845 ± 50 BP (Bacon 1983) and a second 6730 ± 40 BP (Hallet et al. 1997). The second value has reportedly been supported by data from the Greenland GISP2 ice core on the basis of a correlation with a SO4 peak thought to be associated with the eruption (Zdanowicz et al. 1999:623). The usefulness of the Mount Mazama eruption event as a regional time marker is illustrated in the determination that the Kennewick, Washington human skeleton was associated with sediments below the Mazama ash lens. On this basis, the age of 8410 ± 60 BP (Table 5), assigned to Kennewick on the basis of a direct 14C age determination on bone (Taylor et al. 1998), is consistent with its stratigraphic relationship to the Mount Mazama event.

Southwestern United States In the early part of the 20th century, the American Southwest became a focal point of New World archaeological studies. An important factor was that the region enjoyed one of the earliest applications of stratigraphic excavation strategies in the New World combined with typological and seria- tion approaches to the temporal analysis of ceramics and other artifact types. Several other factors Contribution of Radiocarbon Dating 7 contributed to this status, but, of these, probably the most important was that for North America north of Mexico the region enjoyed the most secure prehistoric chronometric framework in the pre-14C period. This was due, to a considerable extent, to the extensive and sophisticated development of dendrochronological applications. Dendrochronology was developed and applied in the Southwest by an astronomer, Andrew Ellicott Douglass, more than two decades prior to the development of the 14C method. Because of the effec- tiveness of dendrochronological approaches for the last two millennia—the period beginning with Basket Maker II on the Colorado Plateau and Mogollon I for central Arizona and New Mexico—14C data was not aggressively pursued for Pueblo, Mogollon and Hohkam sites and materials. However, 14C data were employed in Southwestern subregions where dendrochronology could not be reliably extended—such as the southern Arizona Desert—and for “Archaic cultures” in excess of about 2000 BP (Rohn 1978). Of early significant interest was the Chicago 14C date of about 5900 BP determined on charcoal from a lower level in Bat Cave, New Mexico, purportedly associated with early forms of domesticated maize (Dick 1965). From the beginning, the results were suspect or, as Frederick Johnson termed it, “tenuous,” because of problematic associations, even suggesting that the date should not have been published (Johnson 1955:154). Since that time, attempts to date the arrival of maize and other cultigens in the Southwest have been controversial (Simmons 1986). The development of AMS 14C technology applied resulted in a set of results that have been obtained directly on the cultigens rather than on associated organics. An excellent example of the effects of direct dating of the domesticates in contrast to earlier 14C dates on associated organics is available from the experiences at the Fresnal Shelter in southern New Mexico (Tagg 1996). Minnis (1992), and Hard and Roney (1998) recently assembled the results of such direct dating at sites in Arizona, Utah, New Mexico, and Chihuahua, Mexico. Currently, the earliest reliable evidence for cultigens in this region is about 3000 BP with maize, beans, and one species of squash arriving first.

Mesoamerica Other than the southwestern United States, ancient Mesoamerica—a region in pre-Hispanic times that encompassed the southern two thirds of the modern state of Mexico and most of what is now central America—was the object of the most intensive study by a number of archaeologists and other scholars from universities, museums, research foundations both the United States and Europe. The initial major attraction beginning in the 19th century was a series of sites particularly in the Maya region of the Yucatan peninsula containing extensive ruins of large monumental structures (e.g. Tikal and Copan), in the Valley of Mexico (e.g. Teotihuacan), and Valley of Oaxaca (e.g. Monte Alban). As already noted, the Yucatecan Maya area was the only region in the pre-Columbian New World with a fully formed writing and complex long count calendar system and an extensive inscrip- tional textual corpus. In the case of the Maya calendar, the long count system involved a continuous recording of days from a fixed zero point. Among the Maya city states of the Yucatan Peninsula, hieroglyphic texts and calendar notations were inscribed on stela and on various structural elements of buildings within temple complexes. Long count dates were extensively employed in Maya archaeology to associate ceramic types with architectural phases for the Maya Classic period (Fedick and Taube 1992). A long-standing question was the correlation of the pre-Hispanic lowland Maya long count calendar with the Western calendar. At the time of the introduction of the 14C method, the majority of Mayan scholars had tentatively accepted, from a lengthy list of correlation formulas, the “GMT” (Goodman-Martinez-Thompson) 8 R E Taylor correlation as best fitting the available evidence, although a correlation formula devised by H J Spinden continued to be cited (Thompson 1960:306–9). As summarized in Table 2, the first 14C determination bearing on the Maya correlation problem was obtained by the Lamont 14C laboratory on a sample of wood extracted from a wooden lintel inscribed with a Maya long count calendar notation from the Classic Maya site of Tikal in Guate- mala (Kulp et al. 1951:566). Surprisingly for many Mayanists, rather than supporting the GMT correlation, the 14C value on the lintel supported the Spinden correlation scheme. This correlation calculated dates exactly 160 yr earlier than did the GMT formula. A second 14C determination on another inscribed wooden lintel from Tikal bearing the same long count data as in the first test was undertaken at Chicago by Libby and also supported the Spinden correlation (Libby 1954:740). These results were criticized on the basis that they were derived from samples from existing museum collections whose size had been reduced for transport and thus had lost their outside rings (Satterth- waite 1956). The result of these initial results was the first intensive dating program undertaken by a 14C laboratory in conjunction with an major archaeological excavation—in this case at Tikal from 1955–1970 by the University Museum of the University of Pennsylvania. The first topic addressed was the 14C dating of lintels of temple structures on which were inscribed Maya long count calendar dates. In selecting these samples, there was an explicit recognition of an problem with wood samples containing a number of rings. The issue was first labeled the “post-sample-growth (or inner wood) error” and then renamed, more appropriately, the “pre-sample growth error” (Ralph 1971:4). New collections of wood samples with due consideration of the problem of missing wood were obtained during the University of Pennsylvania excavations. In a critical interpretation of the resultant 14C data, involving more than 100 determinations obtained by the University of Pennsylvania and UCLA laboratories, the pre-sample growth factor was employed to explain the earlier determinations. By this time, there was the beginnings of an understanding of calibration problems for 14C values. Fortunately, early tree-ring data indicated a close correspondence between 14C and solar time for the critical period at issue (Satterthwaite and Ralph 1960; Ralph 1965). Most, but not all (e.g., Andrews 1978), Mayanists accepted the new 14C results as strongly supporting the GMT correlation (Kelley 1983). In the mid-1960s, the pre-sample growth factor was also invoked to resolve problems of correlating 14C determinations on charcoal with the apparently well-known chronological sequence for the Classic period at Teotihuacan in central Mexico (Kovar 1966). 14C dating was also extensively employed to provide temporal resolution for what some archaeologists considered the “mother culture” (cultura madre) of ancient Mesoamerican civilization—the Olmec—associated with sites such as La Venta and Tres Zapotes located in the Gulf Coast region of Mexico. Others viewed the Olmecs as contemporaneous with or even to postdate the Classic Maya. A series of 14C values obtained from La Venta demonstrated clearly that the Olmecs at this site pre- dated the Maya by many centuries (Drucker et al. 1957:265). A suite of 14C values obtained during studies at the site of San Lorenzo documented an even earlier Olmec phase reaching back to at least 3000 BP (Coe and Diehl 1980:395–6). The question of the origins of plant domestication in the New World had been long been centered on Mesoamerica in light of the fact that the wild precursors of the major domesticates—and especially Zea mays—were generally considered to be indigenous to that region. A major excavation in a number of caves and rock shelters in Tehuacan Valley in central Mexico in the 1960s specifically directed at elucidating the course of plant domestication in the region revealed a long stratigraphic record covering most of the Holocene (Byers 1967–1972). Contained in these deposits at various levels were fragments of a number of plant domesticates. A large suite of 14C dates primarily on Contribution of Radiocarbon Dating 9

Table 2 14C data associated with the correlation of the Maya and Western calendars A. Initial measurements Lamonta Tikal Structure 10b Spinden = AD 481; GMT = AD 741 L-113, 1470 ± 120 BP (AD 481 ± 120)c L-113 bis, 1502 ± 60 BP (AD 456 ± 60)d,e Chicago f Tikal Temple IVg Spinden = AD 481; GMT = AD 741 C-948, 1485 ± 120 BP (AD 469 ± 120)h C-949, 1521 ± 170 BP (AD 433 ± 170)h (Weighted average = AD 451 ± 110)h B. Recognition of “presample tree ring growth error” problem Pennsylvaniai Tikal Temple IV Spinden = AD 435; GMT = AD 712 Average 13 Temple IV 14C values = 1213 ± 34 BP (AD 746 ± 34)j Tikal Temple I Spinden = AD 435-452; GMT = AD 695-712 Average 7 Temple I 14C values = 1275 ± 37 BP (AD 684 ± 37)j Tikal Structure 10 Spinden = AD 481; GMT = AD 741 P-293, 1353±57 BP (AD 606 ± 57)j,k UCLAl Tikal Temple IV Spinden = AD 435; GMT = AD 712 Average 2 Temple IV 14C values = 1238 ± 30 BP (AD 712 ± 30)m Tikal Structure 10 Spinden = AD 481; GMT = AD 741 Average 5 Structure 10 14C values = 1344 ± 45 BP (AD 606 ± 45)n aKulp et al. (1951) bFrom American Museum of Natural History, New York c“AD” value calculated by subtracting BP value from 1951 dBroecker et al. (1959) eRecount by gas counting of solid carbon sample used for L-113. “AD” value calculated by subtracting BP value from 1958. fLibby (1955:131) gFrom Ethnographical Museum, Basel, Switzerland h“AD” value calculated by subtracting BP value from 1954 iSatterthwaite and Ralph (1960) j“AD” value calculated by subtracting BP value from 1959 kSame wood as used for L-113 lFergusson and Libby (1963:13–14) mDuplicates of 2 Pennsylvania Temple IV wood samples nDuplicates of 5 Pennsylvania Structure 10 wood samples 10 R E Taylor associated charcoal provided the chronology for development of domesticated species (Johnson and Willis 1970; Johnson and MacNeish 1972). Charcoal 14C values were used to assign age to the individual fragments of early maize as early as 7000 BP. Concerns regarding the association of charcoal with the maize fragments were later addressed when AMS-based 14C measurements on the maize specimens themselves determined that there was a considerable range in the ages of the individual specimens; the oldest exhibited an age of about 4700 BP (Long et al. 1989).

PALEOINDIAN CHRONOLOGY One of the longest running and acrimonious debates in New World archaeology concerns the nature and timing of the peopling of the Western Hemisphere (Stanford 1982; Irving 1985; Taylor 1991). There is no question that the advent of 14C dating transformed discussions concerning dating frameworks for Paleoindian sites and contexts (Wilmsen 1965). The history of the application to 14C in Paleoindian studies provides a classic case study of the great impact that the 14C method made in New World archaeology. The context for 20th century discussions of this issue had their origins in the 1920s and 1930s with the general acceptance of the direct association of two types of fluted projectile points—known as Clovis and Folsom from their type sites—with skeletal remains of an extinct North American mega- fauna, particularly bison and mammoth. These discoveries convinced the professional archaeological community that human populations had entered the New World sometime between 10,000 and 25,000 yr ago (Wormington 1957). In the pre-14C era, the range in age assigned to the entry resulted primarily from differences among geologists and others as to the dating of the final phase of the Wis- consin glaciation and the disappearance of the Pleistocene fauna in North America. Also, on stratigraphic criteria, Clovis was demonstrated to predate Folsom but the temporal offset was unknown (Meltzer 1989). With the advent of 14C dating, one of the first issues for which Chicago 14C dates were obtained was the resolution of the chronological status for the New World Paleoindian period. In pursuing this issue and in later considerations of the question of dating purported pre-Clovis occupations in the New World, a number of major problems that confront the application of 14C in archaeology in general have been addressed and well illustrated. For example, the problem of stratigraphic misattribu- tion of an organic sample (e.g. charcoal) for which a 14C age estimate is obtained and an archaeological or geological context was vividly exemplified in the first 14C age determination on a sample from the Folsom type site in New Mexico. The sample, initially described as charcoal from a fire-pit situated below bison bones and artifacts collected by Harold J Cook in 1933, was dated at 4283 ± 250 BP (an average of two determinations) which generated the comment “surprisingly young” (Arnold and Libby 1950:10). Cook revisited the Folsom site in June 1950 and determined that the “sample had been taken from a hearth in the fill of a secondary channel which had cut through the original deposit of bison bone and artifacts” (Roberts 1951:116). A 14C value of 9883 ± 350 BP (C-558) was subsequently obtained on burned bison bone from what was interpreted as the Folsom horizon at Lubbock Lake, Texas (Libby 1951:293; Roberts 1951:20–1; Haynes 1982:384). It thus appears that the first 14C determination concerned with one of the most controversial issues in American archaeology was deemed unacceptable for what it was supposed to have dated, requiring reinterpretation of the geological context and an additional 14C analysis on a sample presumed to be more directly associated with the cultural or technological tradition for which dating was being attempted. It was subsequently argued that the problem may have not been resolved by the Lubbock Lake values. Geological evidence combined with additional 14C data pointed to the conclusion that the burned bone sample used for C-558 did not, in fact, come from the Folsom levels at the Contribution of Radiocarbon Dating 11

Lubbock Lake site (Haas et al. 1986; Holliday and Johnson 1986). If this is correct, the first 14C age determination securely associated with Folsom materials was obtained on charcoal collected at the Lindenmeier site in Colorado, where a 14C value of 10,780 ± 375 BP was obtained (Haynes and Ago- gino 1960). Over the next four decades, a steadily increasing corpus of 14C determinations were obtained on materials variously associated with a number of Clovis and Folsom sites in western North America. Table 3 represents the results of a critical review of 14C values on various sample materials that are in secure stratigraphic relationship with either Clovis or Folsom occupations (Taylor et al. 1996). Based on this data base, Clovis populations hunted and collected on the North American Great Plains from about 11,600 to 10,900 BP, while Folsom groups flourish from 11,000 to 10,250 BP. It appears that the transition from Clovis to Folsom may have occurred within a period of 100 yr or less (Haynes 1984), although the current suite of dates does not have the precision required to test this assertion (Haynes 1991).

Table 3 Selected 14C dates associated with Clovis and Folsom sites in North America (Taylor et al. 1996). When value cited in the table is an average, the number of 14C values averaged are listed in parenthesis. Clovis sites 14C age (BP) Folsom sites 14C age (BP) Murray Springs (8) 10,890 ± 50 Hanson (4) 10,250 ± 90 Lehner (12) 10,940 ± 40 Blackwater Draw (5) 10,290 ± 90 Anzick 10,940 ± 90 Carter/Kerr McGee 10,400 ± 600 Dent 10,980 ± 90 Lubbock Lake 10,540 ± 100 UP Mammoth 11,280 ± 350 Indian Creek/1 10,630 ± 280 Lange/Ferguson 11,140 ± 140 Owl Cave 10,640 ± 85 Colby 11,200 ± 220 Lindenmeier (3) 10,660 ± 60 Domebo 11,480 ± 450 Agate Basin (2) 10,700 ± 70 Blackwater Draw (3) 11,300 ± 240 Folsom (6) 10,890 ± 50 Aubrey (2) 11,570 ± 70 Indian Creek/2 10,980 ± 150

The central influence of 14C dating on New World Paleoindian studies can be seen most clearly when it is noted that in several cases, a single 14C age determination was the most important factor in directing great attention and relatively massive amounts of resources toward specific sites and regions purported to contain evidence of Paleoindian materials presumed to be older than Clovis. For example, in the early 1960s, the decision to undertake the large scale excavations at Tule Springs, Nevada (Wormington and Ellis 1967) was stimulated in large part by a single 14C determination of >23,000 yr (C-914) obtained by Libby’s group (Libby 1952:121) on what was characterized as charcoal recovered from what had been labeled a “hearth-like feature” by excavators who associated its occurrence with sediments containing the bones of extinct fauna (Harrington 1954; Harrington and Simpson 1961).

14 +3000 In another instance, a C age of 27,000 /−2000 BP (Irving and Harrington 1973) was obtained on an inorganic fraction of a bone implement recovered from a locality in the Old Crow Basin, Yukon Territory, Canada in the early 1970s (Irving 1985:547). At both Tule Springs and in the case of the Old Crow artifact, subsequent 14C data determined that the original age assignments for the cultural materials had been greatly inflated. In the case of the Tule Springs materials, the only uncontested artifacts recovered during extensive excavations carried out in the early 1960s were associated with sediments dating in the 10,000–11,000 BP range. The features originally labeled as “hearths” con- 12 R E Taylor taining “charcoal” were determined to be concentrations of decayed plant remains associated with water channel or spring deposits (Haynes et al. 1966; Haynes 1988). As for the Old Crow implement, an organic fraction of the bone later yielded a 14C value of 1350 ± 150 yr (Nelson et al. 1986). Another type of contamination issue was associated with the 14C dating of “charcoal” materials from the site of Lewisville in Texas. The apparent charcoal had been recovered in the early 1950s from a series of hearth-like features containing extinct fauna that included mammoth and a Clovis-type point. The 14C values ranged between >37,000 and >63,000 BP (Crook and Harris 1958). It was initially suggested that the charcoal derived from packrat middens (Heizer and Brooks 1965). How- ever, renewed excavation in the late 1970s determined that the supposed charcoal used for the 14C determinations was actually lignite, a form of coal. Studies associated with the Tule Springs, Old Crow, and several other sites illustrate the continuing efforts to bring evidence to bear that would support the validity of the view that human populations occupied the Western Hemisphere prior to the appearance of the Clovis hunters and gatherers. The so-called “Clovis boundary” has been continuously assaulted by a minority of New World archaeologists over several generations and have spurred archaeological investigations from Alaska to Tierra del Fuego. However, of the more than 100 sites in North America that have been reported to contain evidence of “pre-Clovis” occupation, only a relatively small number currently remain under active consideration. Of the remaining alleged pre-Clovis sites in North (Payen 1982; Meltzer 1989) or South (Lynch 1990; Meltzer et al. 1994) America, either the cultural nature of the material or the ade- quacy of the chronometric data associated with the remains, or both, have been questioned. In most cases, 14C age determinations have been and continue to be the crucial arbiter and standard used in documenting the age of a given feature or site locus purported to represent a pre-Clovis occupation. Beginning in the early 1970s, a series of 14C age determinations on human skeletal materials and initial results of the application of two other dating methods—amino acid racemization (AAR) and uranium series (U-series)—to human bone appeared to support a pre-Clovis occupation of the New World. In addition to the 14C-based ages on the Los Angeles (>23,000 BP) and Laguna (17,150 BP) skeletons (Berger et al. 1971), AAR-based age estimates assigned ages of 70,000 yr to the Sunny- vale (northern California) skeleton, >50,000 to the Haverty or Angeles Mesa skeleton (southern Cal- ifornia), 48,000 yr to the Del Mar skeleton (southern California) and, apparently confirming an earlier 14C value, 23,000 yr for the Yuha skeleton, although U-series indicated an age of about 19,000 yr (Bada et al. 1974; Bada and Helfman 1975; Bischoff and Childers 1979). The advent of AMS-based 14C technology made practical an intensive reevaluation of the age estimates obtained on these and other human skeletons. AMS 14C analysis permitted routine 14C analyses of milligram amounts of carbon from highly specific molecular fractions extracted from bone (Stafford et al. 1982, 1990, 1991; Hedges and Law 1989; Hedges and Van Klinken 1992; Taylor 1994, 1997). Table 4 summarizes the results of the direct 14C dating or re-dating by several groups of investigators of bone samples on which previous 14C values had been obtained and/or on which AAR- and U-series-based age estimates had been obtained. In all cases, there was a significant downward revision in the originally assigned ages (Taylor et al. 1983, 1985; Ennis et al. 1986). Table 5 summarizes directly 14C-dated North American human skeletal samples in excess of 8000 BP where there are sufficient technical data available to support the age assignments indicated. Currently, a group of human skeletons recovered from Anzick, Montana with 14C values on amino acid fractions ranging from 10,240 to 10,940 BP—represent the oldest, directly 14C-dated example of a New World Homo sapiens sapiens that have to date appeared in published reports. Contribution of Radiocarbon Dating 13

Table 4 Revisions in age estimates on human bone (except Old Crow) from North America sites of purported Pleistocene age based on AMS 14C determinations and related data [A] [B] Original estimate Revised estimate Skeleton(s)/ artifact Basis Age (14C age) Laboratoriesa Sunnyvale AAR 70,000 3600–4850 UCR/Arizona AMS U-series 8300/9000 6300 UCSD (Scripps)/Oxford AMS Haverty AAR >50,000 4050–5350 UCR [Angeles Mesa] 5200 GX (Geochron) 7900–10,500 UCLA 2730-4630 UCR/LLNL-CAMS AMS 4600–13,500b UCR/LLNL-CAMS AMS 5250 DSIR, New Zealand AMS 15,900b DSIR, New Zealand AMS Del Mar AAR 41,000–48,000 4900b UCSD (Scripps)/Oxford AMS U-series 11,000/11,300 4830 Arizona AMS 1150–5060b Arizona AMS Los Angeles 14C >23,000 UCR/Arizona AMS [Baldwin Hills] AAR 26,000 Taber Geologic 22,000–60,000 3550 Chalk River AMS Yuha 14C 22,000 1650–3850 Arizona AMS AAR 23,000 U-series 19,000 Old Crow 14C 23,000 1350 Simon Fraser/McMaster AMS Laguna 14C 7100 5100 UCSD (Scripps)/Oxford AMS 17,150 >14,800 Natchez Geologic “Pleistocene” 5580 Arizona AMS Anzick Clovis 10,000–11,000 8610–10,680 Arizona AMS Tepexpan Geologic “Pleistocene” 920–1980 Arizona AMS Calaveras Geologic “Pliocene” 740 UCR/Arizona AMS aReferences for all values except Calaveras are cited in the caption for Table 25.5 in Taylor (1992); Calaveras data from Taylor et al. (1992) bExperimental osteocalcin fraction with anomalous 14C values (Burky et al. 1998)

Purported pre-Clovis sites that continue to be seriously examined by New World archaeologists have associated with them extensive suites of 14C determinations. In North America, the Meadowcroft Rockshelter in Pennsylvania, a deeply stratified site excavated in the mid-1970s, documented by 52 14C determinations, has been strongly advanced as a well-documented pre-Clovis occupation. Exca- vators initially pointed to a 14C value at about 17,000 BP obtained on what was described as a carbonized fragment of cut bark-like material in the lowest cultural unit (Adovasio et al. 1978). More recently, “applying the most conservative interpretation of the [14C] data,” support for human occupation at Meadowcroft not later than about 14,000 BP has been advanced (Adovasio et al. 1998). A vigorous debate has been carried out (Dincauze 1981) that, on one hand, has raised questions about the potential contamination of samples since a coal-like deposit (vitrinite) is exposed in the rock shelter (Haynes 1980; Mead 1980; Tankersley and Munson 1992) and, on the other, a vigorous defense of the validity of the 14C dates and their association with cultural materials (Adovasio et al. 1980, 1992, 1998). 14 R E Taylor

Table 5 14C-dated early Holocene human skeletal samples from North America Site Sample/fraction 14C age (BP) Anzick, Montanaa Glycine 10,940 ± 90 (AA-2981) Glutamic acid 10,820 ± 100 (AA-2979) Hydroxyproline 10,710 ± 100 (AA-2980) Gelatin (untreated) 10,500 ± 400 (AA-313B) Alanine 10,370 ± 130 (AA-2982) Aspartic acid 10,240 ± 120 (AA-2978) Buhl, Idahob Total acid insoluble organics 10,675 ± 95 (BETA-43055/ETH-7729) Mostin, Californiac Total acid insoluble organics 10,470 ± 490 (UCLA-2171) On-Your-Knees Cave, Prince Gelatin (XAD treated) 9730 ± 60 (CAMS-29873) of Wales Island, Alaska Gordon Creek, Coloradod Total acid insoluble organics 9700 ± 250 (GX-0530) Spirit Cave, Nevadae Total amino acids 9350 ± 70 (UCR-3261-4/CAMS-12353) (hair, bone, fiber) 9360 ± 60 (UCR-3261-2/CAMS-12354) 9410 ± 60 (UCR-3324-1/CAMS-24194) 9430 ± 60 (UCR-3260/CAMS-12352) 9430 ± 70 (UCR-3323/CAMS-24199) 9450 ± 60 (UCR-3261-2/CAMS-14224) 9460 ± 60 (UCR-3324-2/CAMS-24197) Wizard Beach, Total acid insoluble organics 9515 ± 155 (GX-19422) Pyramid Lake, Total amino acids 9110 ± 60 (UCR-3445A/CAMS-26369) Nevada 9210 ± 60 (UCR-3445B/CAMS-26370) 9250 ± 60 (UCR-3445C/CAMS-28124) La Brea, Total amino acids 9000 ± 80 (UCLA-1292B) Los Angeles, Californiaf Kennewick, Washingtong Total amino acids 8410 ± 60 (UCR-3476/CAMS-29578) aStafford et al. (1990); bGreen et al. (1998); cKaufman (1980); dBreternitz et al. (1971); eKirner et al. (1997); fBerger et al. (1971); gTaylor et al. (1998).

Pendejo Cave, New Mexico, with an extensive suite of 14C values documenting more than 55,000 yr of stratified sediments, has been also advanced by its excavators as a site where pre-Clovis occupation has been demonstrated (Chrisman et al. 1996). This view is based, in part, on human skin imprints on clay from different sediment units ranging in age, based on associated charcoal, from 12,000 to >35,000 BP, and the occurrence of a purported human hair samples directly 14C dated at about 12,000 BP. Other investigators have raised a number of questions concerning the relationship of the imprints to the dated sediments and the attribution of the hair as human (e.g. Taylor et al. 1995). In South America, a set of 14C values have been obtained on various organics from the site of Monte Verde in Chile (Dillehay 1989, 1997). In 1997, a senior group of Paleoindian specialists evaluated the archaeological and geological evidence at the site and reported that the series of 14C values were considered to provide a valid indication of the age of the associated artifacts and other cultural features (Adovasio and Pedler 1997; Meltzer et al. 1997). Attention was particularly focused in 14C values that indicated that human occupation at Monte Verde had begun not later than about 12,500 BP— approximately 1000 14C yr earlier than the Clovis occupation in North America. Contamination of samples does not seem to be at issue, and suggestions that there is a major 14C reservoir effect in the region around Monte Verde have not been supported by recent measurements of contemporary organics and consideration of potential sources of magmatic CO2 in the region (Taylor et al. 1999). How- ever, in late 1999, several renewed concerns about Monte Verde, including the integrity of the site Contribution of Radiocarbon Dating 15 context, had been expressed (Fiedel 1999) along with vigorous responses by the original site excavators (Collins 1999; Dillehay et al. 1999).

GENERAL IMPLICATIONS Many commentators have noted that the most immediate and obvious impact of the 14C method on the conduct of New World archaeological research in general was the ability of the technique to provide chronometric age estimates using—to a first-order approximation—a fixed-rate temporal scale which transcended local, regional, and continental boundaries. In the words of J Desmond Clark (1979:7), without 14C data, archaeologists would continue to “founder . . . in a sea of imprecisions sometimes bred of inspired guesswork but more often of imaginative speculation.” His comments reflected his experience in African prehistory, but it can be applied accurately as well in New World studies. It can be legitimately argued that temporal intercomparibility of 14C values were as significant and important a characteristic of 14C values as the degree of their accuracy or precision. Fortu- nately, the temporal framework itself turned out to be amazingly accurate given the number of assumptions that had to hold to rather narrow ranges. A somewhat less recognized contribution of 14C data has been the fact that 14C-based age estimates provide a means of deriving chronological relationships independently of assumptions about cultural processes and totally unrelated to any type of manipulation of archaeological materials (Willey and Phillips 1958:44; Dean 1978:226; Taylor 1978:63). When pressure to derive chronology primarily from the analyses of artifact data was released, inferences about the evolution of human behavior based on variations in environmental, ecological, or technological factors could be aggressively pursued employing an independent chronological framework. In the United States, the rise of the “new archaeology” in the 1970s took place in this context. Louis Binford has reflected that 14C chronology “has certainly changed the activities of archaeologists, so that now, in many ways for the first time, they direct their methodological investments toward theory building rather than towards chronology building” (quoted in Gittens 1984:238). At the inception of the method, Frederick Johnson offered a perspective that was as helpful five decades ago as it is today:

[P]rogress in the development of . . . [radiocarbon dating] depends to a large degree upon the character of the collaboration [between archaeologists and other scientists]. The laboratory procedure involves theories in physics and chemistry which for the most part are outside the experience of almost everyone who has a sample to be dated. On the other hand, the results secured are of little consequence unless they are directly or indirectly related to some stratigraphic sequence. The value of the laboratory results is enhanced by critical evaluation by other scientists. Most particularly, the reverse is true. This involves continual examination of all basic theory and hypotheses by everyone concerned. The future value and usefulness of the method depends in large measure upon the success of continued collaboration between physicists, archaeologists, geologists, botanists, and others [Johnson et al. 1951:62]. This injunction assumed a new significance with the introduction of AMS technology. For archaeologists, the increasing utilization of milligram-size samples required an even more rigorous attention to the evaluation of geological, geochemical, and archaeological contexts of samples. The need for interdisciplinary cooperation and collaboration, particularly with geological specialists, has become even more critical as AMS technology assumes an ever-increasing role in the 14C analysis of archaeological and other late Quaternary paleoecological materials over the next decade and beyond. 16 R E Taylor

CONCLUSION The application of the 14C method to archaeological materials is generally considered to be a water- shed event in the history of archaeology and, in particular, in prehistoric studies (Taylor 2000b). Per- haps the most forceful statement was the view of the late Glyn Daniel that the development of the 14C method in the 20th century should be equated with the 19th century change in the Western world view that accompanied the revelation of the great antiquity of the human species (Daniel 1959:79- 80; 1967:266). Understanding the influence of 14C as being of a “revolutionary” character for aspects of Old World archaeology (Renfrew 1973) is also an accurate representation of its impact on the conduct of New World archaeology over the last half century. That influence has been both obvious and subtle, as well as pervasive and lasting. Radiocarbon dating, which to again quote Frederick Johnson, “dropped like an atomic bomb” on New World archaeology 50 years ago, promises to continue to provide for generations of scholars to come, critical data for those concerned with understanding, from a scientific perspective, the prehistory of the New World.

ACKNOWLEDGMENTS The author wishes to acknowledge the generous assistance of the late Frederick Johnson in understanding initial reactions of archaeologists in the United States. The author also wishes to thank the Gabrielle O Vierra Memorial Fund for support. This is contribution 99/4 of the Institute of Geophys- ics and Planetary Physics, University of California, Riverside.

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THE IMPACT OF RADIOCARBON DATING ON OLD WORLD ARCHAEOLOGY: PAST ACHIEVEMENTS AND FUTURE EXPECTATIONS

O Bar-Yosef Peabody Museum, Department of Anthropology, Harvard University, Cambridge, Massachusetts 02138, USA. Email: [email protected].

INTRODUCTION Half a century since radiocarbon was first used in the archaeology of the Old World, it seems that the expectations of W F Libby may be becoming a reality. In 1952 (Libby 1952:97), he wrote:

Archaeologists, geologists and palynologists are continually searching for the means to improve methods of counting time. The . . . relative chronologies lack precision and direct correlation with the calendar, except when they may be checked with, . . . for example, the calendar based on tree-ring counts. Two achievements that have gone some way towards realizing this goal are the use of accelerator mass spectrometry (AMS) techniques (e.g. Taylor 1997) and of calibration curves (e.g. Stuiver et al. 1993, 1998). We are still only at the threshold of seeing the impact of these two crucial advances on some strongly debated archaeological issues. Since the end of the 18th century, some basic questions that cannot be answered without accurate dates have been at the heart of archaeological research. Practice has demonstrated that the long sequence of human evolution, from the time hominids created durable remains, the early colonization of Eurasia or even the first dispersals of Modern humans, are beyond the upper reach of 14C techniques. During the last five decades, traditional 14C dating techniques have made numerous contributions to the archaeology of the Old World. These are evidenced in a vast literature that reports and discusses the evolution of social and cultural entities recorded from over the last 40,000 years. Terminologies may vary across Eurasia and Africa, but in the most encompassing definitions, this is a world that shifted from foraging lifestyles to farming and herding modes of production, which were then followed by the emergence of urbanism and the ensuing industrial revolution. During these 50 years, archaeologists, geoarchaeologists, and archeobotanists have used the services of an ever-increasing number of 14C laboratories. In many of them, one notes a growing awareness of the need for the active participation of an experienced field archaeologist throughout the entire process, from collecting the samples and the gathering of relevant information, through laboratory operations, and the final evaluation and write-up of the results. While all this is known to the readers, and the contributions that are being made to various archaeological questions are important, there are, in the author’s view, two major concerns in Old World archaeology that are of common interest to a majority of archaeologists and world historians. The advancements in dating these past events or processes will have a far-reaching impact on the interpretation of cultural history. The two main problems are the transition from the Middle to the Upper Paleolithic, a cultural revolution which has also been labeled a “creative explosion” (Pfeiffer 1982), and the origin of plant cultivation in the two presumed centers of early agriculture, namely, the Levant in Western Asia and the middle Yangtze region of China. Precise dating of archaeological contexts and assemblages of plants and animal bones derived from well-excavated sites in these two centers will undoubtedly facilitate the resolution of long-lasting debates concerning the “where” and “when” issues of these events. The more controversial aspect of both inquiries, the “why” question, will undoubtedly

23 24 O Bar-Yosef remain open to scholarly opinions as diverse as there are approaches to world cultural history. Each of these major transitions is summarized below, followed by a brief discussion incorporating future expectations.

The Middle to Upper Paleolithic Revolution Almost no one is seeking the origins of the Middle/Upper Paleolithic revolution in Western Europe, although everyone, including the media, is using the archaeological record from this region to characterize the differences between two populations—the Neandertals and Cro-Magnons. Most writers who present their views on this transition consider it to be a technological and cultural revolution (e.g. Gilman 1984; Gamble 1986; Mellars 1989; White 1989; Stringer and Gamble 1993; Mellars 1996a, 1996b; Mithen 1996; Marshack 1997; White 1997). A few follow the suggestion (Klein 1995, 1999) that it was triggered by a neurological change in the “near-Modern Humans” some 50,000 years ago, which has recently gained further support from a genetic study (Quintana-Murci et al. 1999). However, there are others (e.g. Clark 1997; Straus 1997) who regard the transition as a gradual change that took place on a regional scale. Several scholars suggest that the latest West European Neandertals had demonstrated their innovative capacities before encountering the incoming Cro-Magnons. The arguments for one or another of the alternative interpretations rely heavily on the available 14C dates, a proposed synchronization between TL, ESR and 14C dates, and the drive to reach a calendrical chronology (D’Errico et al. 1998; Zilhão and D’Errico 1999a contra Mellars et al. 1999; see also Van der Plicht 1999 contra Van Andel 1998). Elsewhere, I have suggested that by employing models that explain the Neolithic revolution we may gain insights into the techno-cultural revolution that occurred some 50,000–40,000 years earlier (Bar-Yosef 1992, 1994, 1998c). This analytical procedure would be similar to employing studies of the Industrial Revolution as sources for testing hypotheses concerning the Neolithic revolution. The common denominators for all three of these revolutions include the emergence of new technology in a “core area,” and its dispersal (with or without the cultural baggage) by migrating groups, or by diffusion. Study of the historical process can determine “where” and “when” techno-cultural changes occurred and how long it took for the ensuing diffusion, migration, and impacts to affect the neighboring regions. The “why” question remains within the domain of speculation. In all cases, the precise dates play an important role, and it is in this field that the various dating techniques can make major contributions. Currently, there are only a few archaeological indications that East Africa (Ambrose 1998a, 1998b; Klein 1999), South Africa, the Nile Valley (Van Peer 1998), or the Levant (Sherratt 1997) may have been the original locus of the Middle/Upper Paleolithic revolution. Other proposals point in the direction of central Asia or Anatolia (e.g. Otte 1998). The paucity of field research in East Africa and dated sites in the Nile Valley, however, must leave all options open. Most late Middle Paleolithic or Mousterian sites in the Levant and Northeast Africa are dated at 60 to 50/45 ka BP on the basis of thermoluminescence (TL) and electron spin resonance (ESR) measurements as well as 14C dates >46,000 BP (Bar-Yosef et al. 1996; Bar-Yosef 1998a; Van Peer 1998). Culturally, the end of the Levantine Middle Paleolithic is marked by the appearance of Early Upper Paleolithic (EUP) assemblages in several sites (Figure 1). When assemblages of both periods are compared across the chronological boundary, the change seems to represent a technological revolution (e.g. Marks 1993; Bar-Yosef 1998c). The paucity of bone and/or antler objects and the rarity of marine shell beads from EUP contexts have made the lithic assemblages the main source of information. Impact on Old World Archaeology 25

Figure 1 Map of Western Eurasia and North Africa indicating dates of the Latest Mousterian (in boxes) and Early Upper Paleolithic in various regions. The dates are a combination of TL, ESR, and radiocarbon uncalibrated readings.

The image of the pan-Levantine EUP lithic industries is rather complex, mainly due to the small number of sites, the chronological ambiguities (on which future work is required), and the presence of particular local tool types that make long distance correlations uncertain. The main sites are Ksar ‘Akil (Lebanon), Emireh cave and Boker Tachtit (Israel), Umm el Tlel (Syria), and Üçagizli (Turkey) (Garrod 1955; Marks 1983; Ohnuma 1988; Bourguignon 1996; Kuhn et al. 1999). Boker Tachtit, in the Negev Highlands, which dated to 47 and 46 ka BP (Marks 1983), has produced cores and their refitted blanks (Volkman 1983) that demonstrate the change in how the flint knappers conceived the volume of the flint nodule. Levallois points, typological markers of the late Mouste- rian, were now shaped by bi-directional detachments, thus differing from their predecessors. The shift in methods of stone tool production possibly responded to a change in hafting projectiles, and the invention of spear throwers. Other special projectile points are known as Emireh points—the common tool type in Emireh cave and Boker Tachtit. In Ksar ‘Akil, Lebanon, manufacturers preferred simpler points and special scrapers known as “chamfered pieces”, where the working edge was shaped by a side blow (Newcomer 1970; Bergman et al. 1988; Ohnuma and Bergman 1990). Similar tools were found in Abri Antelias, a neighboring site with one Emireh point, and in Abu Halka, some 30 km further north. Interestingly, the EUP of Haua Fteah cave in Cyrenaica (Libya), named the Dabban culture, is also rich in chamfered pieces, 26 O Bar-Yosef although the precise nature of the relationship between the Libyan sites and those in Lebanon is as yet unknown (McBurney 1967). In northeast Syria, the site of Umm el Tlel produced an industry of points and blades made by uni- directional percussion. All the stone tools are, without a doubt, from the Upper Paleolithic, although the special Emireh point and the chamfered pieces are absent (Bourguignon 1996). Interestingly, the AMS date for layer III2A is 34,530 ± 750 BP (Gif A–93216) and the TL date is 36 ± 2.5 ka (Gif A- 93215). Additional assemblages were uncovered in Üçagizli and Kanal caves (Kuhn et al. 1999), where a blade-based industry resembles that of Umm el Tlel, with end scrapers, burins, and retouched blades. The presence of marine shell beads is noteworthy. Two AMS dates of 39,400 ± 1200 BP (AA-27994) and 38,900 ± 1100 BP (AA-27995) place the assemblage from Üçagizli within the range of the EUP industries of the Levant. It is generally agreed that 14C dates earlier than 30,000 BP should be considered as recording minimal ages. However, Van Andel (1998) has suggested that dates older than 38/39 ka BP are again closer to the real ages and do not underestimate the true age, as is the case for dates younger than 30 ka. Van der Plicht (1999) disagrees. Additional uncertainties arise from the use of different laboratories and the possible contamination of charcoal by bioturbation. In this respect, advancements in dating techniques in recent years should allow us to synchronize TL, ESR, and 14C dates from late Middle and EUP sites in the Levantine sequence. Unfortunately, the size of the time difference between the uncalibrated 14C years and TL and ESR years has various estimates. The proposal that 14C dates in this range (earlier than 30 ka) are younger than the TL and ESR ages only by 3–4 ka (Mellars et al. 1999 and references therein), is in need of further testing. In one case, the 14C dates from Umm el Tlel are only about 2 ka younger than the TL date, and lie within the standard deviation of the latter. Another proposal to combine the results of the two dating techniques was undertaken in Kebara cave (Bar-Yosef et al. 1996). TL measurements place the upper part of the Mousterian sequence in Kebara at 48.3 ± 3.5 ka (Valladas et al. 1987), although there are no secure dates for the latest occupation. The EUP assemblages, which are definitely younger than the phase containing the Emireh points, were 14C dated to 43/42 ka. It was therefore suggested that a cautious estimate of 46/45 ka BP for the MP/UP transition seems reasonable, and the gap in the Kebara sequence from 46/45 ka to 43 ka lends credence to the 14C dates for the Boker Tachtit Level 1 (47 and 46 ka; Marks 1983). Another option in dating the boundary between the Middle and the Upper Paleolithic in the Levant is to employ the dates available for the Ksar ‘Akil sequence. Mellars and Tixier (1989), similarly to McBurney in his study of the cave of Haua Fteah (Libya), estimated the rate of sedimentation for this site. Eleven AMS readings of charcoal samples from Ksar ‘Akil, in addition to the previously obtained 14C dates, allowed them to estimate the cultural transition as taking place around 50 ka. Surprisingly, the U-series disequilibrium dates on two bone samples produced earlier, by scientists who cautioned against accepting them without reservation (Van der Plicht et al. 1989), provided similar results. The bone dates are given as “surface” and “bulk” material, and are as follows: for layer XXVI (youngest Mousterian level) 47 ± 9 ka (G-88174S) and 19 ± 5 ka (G-88173B); and for layer XXXII (Mousterian) 51 ± 4 (G-88177S) and 49 ± 5 (G-88178B). The situation in the Taurus and on the Anatolian plateau is poorly known (Yalçinkaya et al. 1993; Otte et al. 1995; Kozlowski 1998), with the exception of the latest Mousterian layers at Karain cave Impact on Old World Archaeology 27

(Antalya province), which were ESR dated to 62.0 ± 10.1 to 71.6 ± 11.4 ka (EU), or 65.5 ± 10.6 to 74.4 ± 11.8 ka (LU) (Çetin et al. 1994). No dates are available for the EUP in this vast region. The state of dating in the Zagros, where several cave sites have been excavated, is not much better (Solecki 1963, 1964; Dibble 1993; Dibble and Holdaway 1993; Solecki and Solecki 1993). The 14C results of 46 and 50 ka from layer D in Shanidar, where several of the Neandertal remains were uncovered, could be argued as simply minimal dates, or as indicating the persistence of the Middle Paleolithic in this mountainous region. The Upper Paleolithic industry known as the Baradostian is dated by a series of readings to 33–28 ka, and in Yafteh cave to the same range (Smith 1986). The absence of the EUP from this site and the other known caves lends temporary support to this interpretation. Further north, in the Caucasus region, similar Mousterian industries seem to be of the same, late age (Kozlowski 1998; Golovanova et al. 1999; Figure 1). Broadening the geographic scope of the present overview, namely, the dating of the late Middle Paleolithic and EUP, introduces additional evidence for what may have been a patchy colonization of Cro-Magnons across Eurasia (Figure 1). In Crimea, producers of the Mousterian industry survived until 29 ka BP, and the early EUP—dated to 30 ka BP—is interpreted as demonstrating a short coexistence of two populations (Marks and Chabai 1998; Rink et al. 1998). In Greece, the late Mousterian is dated to 32–30 ka BP by a series of 14C dates from Theopetra cave in Thessaly, supported by the earliest dates for the EUP in Klisoura cave 1, in the Argolid (Karkanas 1999; Kyparissi-Apostolika 1999; Koumouzelis et al. forthcoming). The late survival of Neandertals is also evidenced in the direct dates of the human bones from Vindija cave in Croatia, which place these relics at 28 ka BP (Smith et al. 1999). On the other hand, an EUP industry known as the Bohunician is dated in Bohemia to 40–36 ka BP (Svoboda and Simán 1989). Further west, the earliest Aurignacian in northeast Spain dates to 40–37 ka BP (e.g. Bischoff et al. 1989, 1994; Cabrera and Bernaldo de Quirós 1996; Straus 1996; Mellars et al. 1999). The persistence of the Mousterian in southern Italy (Kuhn and Bietti, forthcoming) and in Iberia south of the Ebro valley until about 30 ka BP is, in most cases, founded on numerous 14C dates for the late Mousterian and EUP (Raposo and Santonja 1995; Barton et al. 1999; Zilhão and D’Errico 1999a, 1999b; Carbonell et al. forthcoming). Figure 1 presents an overall geographic summary. Although boundaries between the Neandertal and Cro-Magnon territories are not marked, the question is raised of whether Neandertal populations across southern Europe continued to be in touch with each other after 40–38 ka BP, or became isolated groups. Small populations, as modeled by Zubrow (1989), if not intermarrying with incoming groups, could disappear within a relatively short period. There is little doubt today that the rapid cultural changes through the Upper Paleolithic times reflect the results of a major revolution. There were significant technological and social changes, but they are not easy to decipher, due to the grosgrain of chronological resolution as presented above. As with other revolutions, the nature of the changes is better documented after a certain lapse of time, when the new cultural expressions stand in contrast to those of pre-revolutionary times. In the case of the European sequence, the proliferation of lithic blade industries, antler and bone tools, mobile art objects, and cave art (in the Franco-Cantabrian region) gives a good example. In the Near East— despite the more ephemeral character of the Upper Paleolithic sites—the evolved blade technology, the appearance of grinding tools, and the modest use of bone, antler and marine shells mark the cultural shift. That the change was rapid is clearly demonstrated by the radiometric scale. From 270/ 250 ka through 48/46 ka BP, Mousterian lithic industries were pre-eminent, while from 45/42 ka BP 28 O Bar-Yosef onwards, laminar industries formed the basic stone tool-kits, and involved the use of various raw materials, while the appearance of imagery was seen.

Origins of Agriculture in Western Asia The agricultural revolution, or as it is known in the archaeological literature, the “Neolithic Revolu- tion”, is a topic that has attracted historians, archaeologists and botanists since the 19th century. The impact of plant cultivation by sedentary communities on human diets and rates of reproduction is considered the crucial threshold that caused rapid population growth in many parts of the world during the Holocene (e.g. Bar-Yosef 1998c; Cohen 1977; Harris 1998a, 1998b; Smith 1998). As with all important past revolutions, the emergence of plant cultivation some 11,000 years ago, followed by animal domestication, is evaluated on the basis of its outcome. Gradualists see the cultural and socio-economic changes as a slow process that took thousands of years to complete. Others view the change as radical and rapid. The question of “why” a particular change took place is often the most debated. Once there are records based on field and laboratory observations, however, archaeologists tend to agree on the “when” and “where” aspects of the studied revolution. It is in both these aspects that AMS 14C measurements, especially when calibrated, can revolutionize past understandings and pose additional challenges. The Fertile Crescent of western Asia and the Yangtze River valley are considered the two oldest centers of the transition to agriculture in the Old World (Smith 1998). Like other major revolutions in history, the Neolithic revolution began in a core area. The locus of early cultivation practiced by Neolithic villagers is still uncertain. Past hypotheses placed incipient farming in the natural habitat of cereals (Braidwood 1975), or at the edges of the main distribution of the progenitors, namely, in the marginal belt where foragers experienced decreasing yields in plant food resources in the face of prolonged worsening of environmental conditions (e.g. Binford 1968; Flannery 1973). Archeobotanical evidence of carbonized plant remains from Neolithic sites in the Levant points to the location in which cultivation began (e.g. Harris and Hillman 1989; Miller 1992, 1997; Hillman 1996; Heun et al. 1997; Harris 1998a). There is little doubt today that systematic cultivation and har- vesting in the same fields year after year resulted in the domestication of plant species (Zohary 1989; Zohary and Hopf 1994; Bar-Yosef and Meadow 1995; Harris 1996a, 1996b, 1998b). Once communities of cultivating foragers were established, the domestication of goats and sheep was initiated (Legge 1996), followed later by cattle and pigs (Uerpmann 1989). The search for the earliest farming communities began with the pioneering project of R Braidwood (1952, 1973, 1983), which targeted the hilly flanks of the Zagros, where wild cereal species grow today. His choice relied on botanical surveys that mapped the distribution of the various Cerealia species across western Asia (Harlan and Zohary 1966; Harlan 1977; Zohary and Hopf 1994). Unfor- tunately, at the time these surveys were conducted, the impact on the vegetation of Terminal Pleis- tocene–Early Holocene climatic fluctuations was not taken into account, a fact realized only later (Wright Jr 1993). In the late 1990s, archaeologists and archeobotanists began to create an evolutionary scenario based on various kinds of data sets. First, information retrieved from pollen cores and the deep-sea cores from the Eastern Mediterranean provides the distribution of the paleo-phytogeographical belts (Van Zeist and Bottema 1991; Roberts and Wright Jr 1993; Baruch 1994; Bottema 1995; Rossignol- Strick 1995, 1997; Hillman 1996). Adopting the correction for hard-water effects on 14C dates in inland lakes, proposed by Rossignol-Strick (1995), established sound correlations between marine Impact on Old World Archaeology 29 and terrestrial pollen cores. According to this scheme, the Younger Dryas is signified by the abundance of Chenopodiaceae, followed by an increase in deciduous oak pollen that marks the early millennia of the Holocene and reflects the increase in annual precipitation. Second, there is a general agreement on the identification of the Younger Dryas, whether in marine sediments, lake cores, or speleothems (Wright Jr 1993; Rossignol-Strick 1995; Hillman 1996; Land- mann et al. 1996; Bar-Mathews et al. 1997, 1999; Lemcke and Sturm 1997; Fontugne et al. 1999; Frumkin et al. 1999). The conditions prevailing during the Younger Dryas are crucial in interpreting the archaeological remains, and, unfortunately, the dating of this period in the Near East is not without difficulties. According to the ice cores, the Younger Dryas lasted from 12.9 to 11.6 ka (Alley et al. 1993; Mayewski et al. 1996), while in the varve sequence of Lake Van in eastern Turkey (Lemcke and Sturm 1997), this cold and dry period was longer by around 800 years. The third source of data is carbonized plant remains, which indicate “where” within the region various seeds were collected (Hillman et al. 1989; Hillman 1996; Kislev 1997). The seeds, if in secure archaeological context, often provide more precise dates for the “when” question, especially through AMS measurements. Although the number of directly dated seeds is currently rather small, the growing awareness among archaeologists and archeobotanists that this is the way forward facil- itates the testing of several hypotheses in the near future. Meanwhile, available charcoal dates already provide an interesting picture, whether at the level of a particular site, or across a micro- region such as the Jordan Valley (Figures 2–5). A brief summary of the paleoclimatic sequence of the Terminal Pleistocene, following the Last Gla- cial Maximum (LGM), would begin with an increase of annual precipitation and a slow temperature rise from around 15,500 BP. The typical eastern Mediterranean cycle of wet, cold winters and dry, hot summers was established during this period and not later, as was suggested previously (McCor- riston and Hole 1991; Wright Jr 1993). The rapid expansion of oaks (mainly the deciduous Q. ith- aburensis), olives, and pistachio (which is always misrepresented in the samples due to low pollen production), as well as the cereals, which were present in the region from 19,000 BP, testify to this annual climatic pattern (Baruch and Bottema 1991). The ensuing changes are recorded in the terrestrial pollen diagrams and were plotted fairly recently by Hillman (1996) as two vegetation maps for Western Asia, for 13 and 11 ka BP (uncalibrated), respectively. These maps, although based on the terrestrial pollen cores (see above), demonstrate the expansion of three plant associations as follows: 1) forests and woodland in the Mediterranean coastal plain and hilly ranges, 2) oak-terenbinth, a mosaic of woodland and open areas dominated by annual grasses further inland, and 3) terenbinth-almond woodland-steppe that phases into the desertic Saharo-Arabian associations (Zohary 1973). The natural stands of wild cereals are within the last two belts and often appear as grasses in the oak parkland. The expansion of the Mediterranean vegetation and especially of the natural habitats of the cereals resulted from increases in rainfall and temperatures. The prevailing climatic conditions of the Bölling/Allerød (ca. 15,000–13,000 cal BP) favored the growth of C3 plants (Sage 1995), used by Levantine foragers from at least 19,000 BP onwards (Kislev et al. 1992). The improved conditions seem to have served as an impetus for intensive exploitation of cereals and legumes, as well as fruit trees and acorns. The archaeological evidence indicates an increase in sedentism, a broad-based economy relying on extensive exploitation, and the emergence of a complex hunter-gatherer society known as the Natufian culture (Figures 2–3; Henry 1989; Belfer-Cohen 1991; Bar-Yosef 1998b). 30 O Bar-Yosef

Figure 2 Calibrated generalized chronology for the Terminal Pleistocene and Early Holocene of the Levant, indicating both uncalibrated and calibrated BP dates. The figure demonstrates that the Earliest Neolithic actually began during the Younger Dryas.

The proliferation in recent years in the number of 14C dates reveals that the dry and cold climate of the Younger Dryas was probably the main cause for the initiation of systematic cultivation (Bar-Yosef and Belfer-Cohen 1992; Moore and Hillman 1992; Bar-Yosef 1998a; Hole 1998). The crisis of the Younger Dryas, which lasted for about 1300 ± 70 yr (Alley et al. 1993; Mayewski et al. 1993), was due to its effect on the vegetation of Western Asia. It stopped the advance of the woodland into higher altitudes inland (in the Taurus and Zagros Mountains) and reduced the belt of oak and terenbinth. This reconstructed scenario is supported by the identification of carbonized plant remains from Mureybet (Van Zeist 1986; Van Zeist and Bakker-Herres 1986) and Abu Hureyra (Hillman et al. 1989; Moore and Hillman 1992; Figures 3–5), where cereals decreased; and Halan Çemi, which, on a more eastward tributary of the Tigris, by that time had no cereals present (Rosenberg et al. 1995). Human acts are seen as the results of social decisions. It is hypothesized that the determining decision in favor of intentional cultivation was taken in the face of decreasing yields of cereals in the wild Impact on Old World Archaeology 31 stands, in combination with the recognition that other economic solutions, such as becoming more mobile, given the regional population densities, were not the optimal way to minimize risk. The assumed depletion in the natural yields is a testable hypothesis. It relies on the slight decrease in atmospheric CO2 values during the Younger Dryas as the limiting factor in the distribution of the oak- terenbinth belt, and in particular, in the declining annual returns among C3 plants such as the cereals, which had become a major source of carbohydrates for Levantine foragers (Bar-Yosef and Meadow 1995). The paleo-phytogeographical reconstruction points to a relatively narrow strip in the Levant in which the progenitors of most cereal species grew (Hillman 1996). This belt, although a series of delineated areas (Van Andel and Runnels 1995) also known as the “Levantine Corridor,” became the locus in Western Asia in which the first agricultural communities were founded (Figure 4; Bar-Yosef and Meadow 1995; Bar-Yosef 1998c). The decision for economic change was probably not an easy one. It entailed the re-organization of the division of labor, seasonal scheduling of work, allocation of energy expenditure at different times of the year, and the like. However, the stable provision of a sta- ple food meant an increase in the fertility rates, which, despite rising infant and toddler deaths (evidenced in burials), resulted in relatively rapid population growth (Bentley et al. 1993; Bentley 1996).

The return to increasing CO2 levels and higher annual amounts of precipitation during the early Holocene provided conditions suitable for successful cultivation (e.g. Araus et al. 1999). Hence, early farming communities—known archaeologically in the Levant as Pre-Pottery Neolithic A (PPNA)—and particularly their descendants—during the Pre-Pottery Neolithic B (PPNB)—flour- ished (Figure 5). The ensuing off-shoot villages resulted in emigration and demic-diffusion into Europe, the Mediterranean islands, northeast Africa, and southern and central Asia (Ammerman and Cavalli-Sforza 1984; Wetterstrom 1993, 1998; Van Andel and Runnels 1995; Meadow 1998). At the same time, the wetter and warming climate of the early Holocene facilitated the larger geographic dispersal of the wild-cereal progenitors, at later times reaching the current distribution.

Figure 3 Calibrated dates for the Natufian (both Early and Late) and the Harifian (a desertic entity in the Negev and northern Sinai). A general correlation between the onset of the Bölling/Allerød and the emergence of the Natufian culture is suggested, as is the contemporaneity of the Natufian and Harifian with the Younger Dryas. 32 O Bar-Yosef

Figure 4 Calibrated dates of Early Neolithic sites in the Jordan Valley and the neighboring hilly areas indicate that the large communities such as Jericho, Netiv Hagdud, and Gilgal were probably the result of the emergence of intentional cultivation during or immediately at the end of the Younger Dryas.

DISCUSSION In the previous sections, only two issues from the endless number of archaeological investigations were chosen for presentation. In both cases the demand and need for accurate dating have a major impact on the social interpretation of the data. However, there are other domains in which AMS 14C measurements seem to revolutionize our interpretations, and one of these is the study of cave art. This is not only a subject that continues to interest specialists, but is also a topic in art history, and continues to be studied by students of human cognition and its intricate evolution. Even a cursory survey will demonstrate that brain scientists and social psychologists, among others, cite and interpret prehistoric cave art (mostly from the Franco-Cantabrian region) as evidence for symbolic behavior. In addition, mobile objects that fall under the category of imagery are being considered as such (e.g. Marshack 1972, 1997; Donald 1991; Mithen 1996; Conkey et al. 1997; Deacon 1997; Klein 1999). It is, therefore, worth noting that direct AMS dating of samples carefully removed from paintings has enabled investigators to test previous hypotheses concerning their age, and in particular, to confirm that the earliest cave paintings, in the site of Grotte Chauvet, date back to 32–30 ka and in Cosquer Cave to 28–26 ka (Clottes et al. 1995; Clottes 1996a, 1996b). These dates tally well with the even older mobile art objects and body decorations known from the Aurignacian, and support the contention that this culture differs entirely from the Mousterian and thus signifies the techno-cultural revolution of the Middle to Upper Paleolithic. Another well-known historical example is the dating of the famous Shroud stored in the Cathedral of St John the Baptist in Turin, Italy. In this case, the three series of AMS dates carried out indepen- Impact on Old World Archaeology 33 dently by three laboratories support the history of this object as first noted in the mid-14th century AD. The calibrated 14C dates suggested a range of the late 13th to 14th centuries AD (Damon et al. 1989; Taylor 1997). The calibration of conventional 14C ages has already had some major impacts in archaeology. The first all-encompassing attempt to evaluate the impact of the calibration curve on archaeological interpretations was made by Renfrew (1973). In this influential survey, the chronological paradigm of GChilde—which was based on artifact and assemblage correlations across the Mediterranean and Europe—was used, with the Egyptian timetable as a basic yardstick. However, when the available 14C dates for various cultural manifestations from Greece through Britain were calibrated, non-diffu- sionist explanations were put forward. Today, correlations between Egypt and Greece are considered well established. Models based on diffusion and migrations are back in fashion (e.g. Anthony 1990), and like other explanations, propose that the expansion of farming from the Near East to Western Europe can be correlated with the dispersal of Indo-European languages (Renfrew 1987).

Figure 5 The radiocarbon calibrated chronology of Abu Hureyra on the Middle Euphrates River (Moore et al. 1986), from which carbonized plant assemblages were recovered. The dates indicate that the emergence of the farming community was either during or at the beginning of the PPNB.

Chronologies earlier than the third millennium BC in the Near East are dependent on 14C dates. Time estimates employed by archaeologists to evaluate whether a socioeconomic or cultural change was rapid or slow relied until now on non-calibrated 14C dates. Correlations with Ice Core chronology, which is calendrical, require the calibration of dates derived from archaeological contexts. This would, for example, be the only way to test hypotheses that climatic changes triggered cultural changes in a given region. However, everyone who uses the calibration curve is familiar with the existence of “plateaux” when even a date with a rather small standard deviation could indicate several potential calendrical dates (e.g. Hajdas et al. 1995). Unfortunately, the time of the origins of agriculture also seems to coincide with one of these plateaux. 34 O Bar-Yosef

Archaeologists should be able, in forthcoming years, to resolve the issue of chronological ambiguities. A potential way to overcome the problem of a “plateau” in the calibration curve is to obtain past climatic information from well-stratified, dated samples. Previous work has demonstrated that carbonized plants preserve the original ratios of 16O/18O and 12C/13C (Marino and DeNiro 1987; Marino and McElroy 1991). Similar investigations in the Near East provided promising results. For example, wood samples from the first century AD rampart in Masada, or on carbonized cereal grains from PPNB Tel Halula indicate the wetter climate or higher level of water availability during the lifetime of the plants (Araus et al. 1999; Yakir et al. 1994). This approach requires that carbonized seeds be collected with special attention paid to their stratigraphic position from sites that span the time of the Late Natufian and Early Neolithic, that is, from 13,000 to about 10,000 BP (calibrated). The isotopic information from a stratified sequence could be then compared with the climatic curve of the ice cores, although it is expected that the 14C dates will fluctuate between older and younger readings (e.g. Hajdas et al. 1998). Such a research project would force archaeologists to indulge in an as yet very uncommon standard of behavior: that of publishing the sections of the sites and indicating from where the samples were taken (see for example Bar-Yosef et al. 1996). This kind of information, when accompanied by a report on the site’s micromorphology, a study that would clar- ify the amount of disturbance, often of biogenic origins, would enable readers to evaluate the integrity of the so-called “archaeological context” (Courty et al. 1989; Goldberg and Bar-Yosef 1998). The cumulative experience of field archaeologists indicates that “clean” contexts are not easy to trace in Early Neolithic sites, however, given their potential in resolving important historical questions, the additional efforts would be worthwhile. In sum, the last decade of 14C dating has already made a significant impact on archaeological and historical interpretations. In an atmosphere of improved cooperation between scientists and archaeologists, new avenues of research can bring us revolutionary answers to old questions.

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A RADIOCARBON DATABASE FOR SCOTTISH ARCHAEOLOGICAL SAMPLES

P J Ashmore Historic Scotland, Longmore House, Salisbury Place, Edinburgh EH9 1SH, Scotland. Email: [email protected]. G T Cook • D D Harkness Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, Scotland

ABSTRACT. For the majority of dating laboratories and their respective user communities, the journal Radiocarbon is no longer regarded as the medium for primary publication of radiocarbon measurements. In compliance with editorial policy, the emphasis has long since moved towards the publication of research papers on technological enhancements and applications of 14C as well as other cosmogenic isotopes and this has left a requirement for an alternative medium for the publication of date lists per se. In the late 1980s, an International Radiocarbon Data Base was proposed by Renee Kra (then the managing editor) but limitations in computer and communications technologies together with the inevitable financial implications meant that this timely concept could not be taken to completion. In the last year, we have taken advantage of the development of the worldwide web to compile a database of 14C age measurements of a Scottish archaeological nature which can be found at the web address http://www.historic-scotland.gov.uk/.

INTRODUCTION We are all well aware of the breadth of scientific disciplines in which radiocarbon dating has had a fundamental role in advancing knowledge. Indeed, the first paper to be published in the American Journal of Science, Radiocarbon Supplement—the forerunner to Radiocarbon (Olson and Broecker 1959), lists measurements within the fields of archaeology, oceanography, glacial geology, and lim- nology. At this time, the Supplement was to “serve henceforth as the medium for primary publication of all radiocarbon measurements, or at least of radiocarbon date lists” (Deevey and Flint 1959). And so, the journal Radiocarbon became established as the focus for the 14C dating technique and its scientific community. Therefore, this community has been fortunate in that from the outset of the applied chronology, it has benefited from an internationally agreed format for the definition and publication of results policed via a dedicated journal. For approximately the next two decades, the journal continued as a medium for datelist publications although, despite the fact that application of the 14C technique had expanded enormously, the number of published measurements did not increase in parallel. It was becoming obvious that the journal could no longer fulfil its initial perceived role. There was also a gradual shift in emphasis towards the publication of applications and technological innovations and this shift in editorial policy is highlighted by the change in name to Radiocarbon, An International Journal of Cosmogenic Isotope Research, coincident with the movement of the editorial office to the University of Arizona in 1989 and a stated intention to expand the journal’s scientific outlook. In 1987, Renee Kra reported at the Archaeology and 14C Conference in Groningen that less than 10% of known 14C measurements were being published in the journal and that a new vehicle for the dissemination of results was required. This was the first workshop on the International Radiocarbon Data Base (IRDB) (Walker and Kra 1988), a project that Renee pursued vigorously over the next few years (Kra 1988, 1989) to the stage where pilot projects were set up. Unfortunately, the technology of the day, which was very limited by current standards, was such that there were significant cost implications and the IRDB could not be carried through to completion. Nevertheless, the way ahead was demonstrated and the quantum leaps in technology that have occurred during the last decade make the development and worldwide dissemination of database entries a technologically

41 42 P J Ashmore et al. trivial task. We would encourage other laboratories and/or appropriate consortium groups to consider this option if they have not already done so. The date list described here is available at the web address http://www.historic-scotland.gov.uk/, and comprises the age measurements of a Scottish archaeological nature commissioned by Historic Scotland, together with others that have been identified within the published scientific literature, to the end of May 1996.

Content and Arrangement of the Database The tables and columns contained in the database are arranged as follows.

Site name The name of the site Reference The name of the submitter or a bibliographic reference. ‘Forthcoming’ means the site has not yet been published. National grid ref. A six or eight figure national grid reference Calibrated age The Adjusted Age BP converted into a calendar date range using the programme OxCal 2.81 and the 1998 calibration curve. The figures quoted are for a range within which there is a 95% chance that the true age lies. Context, taphonomy, A mixture of comments from the original submission and from subsequent con- and comments sideration. As used here, taphonomy means ‘how the datable material in the sample used for dating got to where it was found on the archaeological site’. Material dated A simple division into charcoal, shell, human bone etc. Lab code The unique code quoted by the laboratory. Each consists of a laboratory iden- tifier and a number. For instance, GU-1000 means age number 1000 from GU (Glasgow University, now housed at the Scottish Universities Environmental Research Centre). Laboratory age BP The conventional (raw) radiocarbon ‘age’ and error (one sigma) as quoted by the laboratory or primary publication. BP means before present where present is 1950 AD. Adjusted age BP Error terms have been adjusted as explained below. Marine shell ages have been adjusted to correspond to terrestrial ages. δ13C Basic radiocarbon theory assumes that there is global uniformity in the natural 14C/12C ratio. This is valid for the well mixed atmosphere and the flora and fauna that it supports provided an allowance is made for the extent to which isotopic fractionation occurs during the assimilation and metabolic fixation of atmospheric carbon dioxide. Although plants obtain their carbon from the atmosphere, the actual 14C activity in them is lower by 3–4% (equivalent to an apparent excess age of between 240 and 320 years). For each sample a δ13C value is determined. This represents the difference in parts per thousand (‰) between the ratio of 13C to 12C in the sample to the ratio in a standard (creta- ceous belemnite, Belemnita americana, from the Peedee formation in South Carolina, known as PDB). The fractionation between 14C and 12C is assumed to be twice that induced between 13C and 12C. A correction factor is determined which normalises all activities to those of wood (with a δ13C of −25‰). Typical values for a range of sample types are as follows: −21‰ for human or animal bone, −25 to −26‰ for charcoal, −28 to −29‰ for peat and 0‰ for marine shell. Because preparation of samples for measurement can sometimes induce a further small fractionation effect, and the quoted δ13C value includes the results of this fractionation, most of the δ13C measurements quoted here are not suitable for inclusion in stable isotope studies.

Some 14C ages in the list have not, so far as we are aware, been published. They are marked “forthcoming”. Many Scottish archaeological 14C ages obtained in 1995 and subsequent years have been published in Discovery and Excavation in Scotland (Ashmore 1996, 1997, 1998; Sheridan 1997b). Database for Scottish Archaeological Samples 43

Understanding of the significance of these ages will be greatly improved once their archaeological contexts have been fully published. Thus 1995–1996 provides a sensible cut-off date, at present, for inclusion of 14C ages in this list.

An Overview of Radiocarbon Research in Scottish Archaeology The impact of 14C dating on our understanding of the prehistory of Scotland has been dramatic. Around 1950, chronologies were based on typology and long distance connections with the Mediter- ranean. The methods used to date sites encouraged the seeking of long distance direct links in a way that is not now found acceptable. Charles Calder, in his discussion of the Staneydale Temple, referred explicitly to Maltese structures (Calder 1952), then supposed to belong to the Bronze Age. While Staneydale, by analogy with the 14C dated house at Ness of Gruting, probably belongs around the end of the third millennium BC or the start of the second (Barcham 1980), the Maltese Temples are generally third millennium in date. There is no known social or cultural link between the Shetland sites of around 2000 cal BC and the earlier Maltese sites. Thus, the lack of an objective dating system independent of superficial similarities between structures distorted interpretations. Around 1950, chronologies had become as short as they were ever to be. The Neolithic of Britain was telescoped by one of the most knowledgeable and well-informed prehistorians of the time into the period after 2000 BC (Piggott 1954:Figure 64). Nor was he alone. For instance, when the prominent Scottish prehistorian, Sir W Lindsay Scott, reported Neolithic pottery from Eilean an Tighe, near the Sound of Harris, he described it as “not later than the mid-second millennium BC” (Scott 1953). Indeed, he ascribed the site to the second millennium BC. It would now be dated by analogy with other sites with similar pottery broadly to the latter part of the fourth millennium BC. In short, 14C shifted the dates for the Scottish Neolithic back 2000 years and extended it three or four-fold, with all sorts of consequences for archaeological interpretation. There seems to have been some suspicion of 14C ages at first. For instance, one of the first archaeological age measurement for Scottish material (BM-441 3514 ± 120 BP) from the British Museum (Barcham 1980), from a large cache of grain at Ness of Gruting referred to above, was long unre- ported. Yet viewed as a whole, the dates from the “heroic age” of 14C dating were often approximately correct, at least if it is accepted that the errors were somewhat underestimated. Some archaeologists sent samples to several laboratories (Renfrew et al. 1976; Renfrew 1989) and their results still make sense today. Indeed the problems with ages obtained in the “heroic age” seem in retro- spect to be due at least as much to poor sampling by archaeologists and in particular the submission of mixed samples (Ashmore 1999) than to faults in laboratories, perhaps with a small number of exceptions (Spriggs and Anderson 1993). That at least is the likeliest explanation for the problematic ages from some sites, such as the intriguing late Neolithic enclosure at Raigmore near Inverness (Simpson 1996:82–3). That said, some very curious results still provide puzzles for archaeologists. For example, markedly different ages from animal bones from one context at the Neolithic houses at Knap of Howar in Orkney of 5706 ± 85 and 4081 ± 65 BP (SRR-347 and SRR-452, respectively) (Ritchie 1983). Since that time, many of the then-operational laboratories seem to have ceased to provide archaeological age measurements. These include Birm (Department of Geology, Birmingham University, England), HAR (Harwell), and NPL (National Physical Laboratory, Teddington, Middlesex). The few that remain in Britain and Ireland are at the forefront of research on calibration and the accuracy and precision of the dating process, together with peers in other countries. That said, and more generally, there is currently an acknowledged tension between the provision of measurements at a price which archaeologists can afford and quality assurance for the whole process of dating from sample 44 P J Ashmore et al. pretreatment onward. The latter continues to be addressed by international inter-comparison studies in which SUERC and Glasgow University have played a leading part. It is essential that such studies be continued and strengthened. Despite the constant improvements in laboratory techniques, dating technology always lags behind archaeological expectations. While analysis of the organic material in each archaeological and paleoenvironmental context is normally restricted to one or two measurements (for reasons of cost) and while precision to within a few 14C decades is the best that can be afforded, many archaeological and paleoenvironmental questions cannot sensibly be asked, let alone answered. Some of the problems are fundamental, relating to the variable proportion of 14C to stable carbon in the atmosphere during the past; others arise because the ages of the samples are often proxies for the age of the context or activity which gave rise to the context. There are, however, problems perceived by archaeologists and paleoenvironmental specialists which can be tackled by simple education. The first of these is a legacy of “heroic age” measurements. These are embedded in archaeological literature and there is a tendency to quote them with unrealistically high confidence, i.e. low numerical error terms. Agreement is required on sensible correction factors for error terms on ages measured in the period before the early 1980s. That requirement has been partly addressed in the database of archaeological age measurements from Scotland contained at the web address quoted above and no doubt, its publication will stimulate useful discussion and further improvements. The second is an archaeological and statistical problem. To overcome the problems of residual, turbated, and intrusive material in a context like a pit or post- hole, large numbers of age measurements for individual entities together with careful statistical analysis will be required. Bayesian techniques (Buck et al. 1996) or, perhaps with a greater prospect of testing competing hypotheses, the “likelihood” approaches advocated by Edwards (1992) should be applicable to modeling the age spectra of organic inclusions in archaeological deposits. Publication of the compendium of Scottish archaeological age measurements will allow a much improved assessment of the basis for current chronological models. For example, there is a tantaliz- ing possibility that the early Neolithic of Scotland can be “debundled”, in the sense that structural and pottery types currently assumed to be roughly contemporary can be shown to be chronologically distinct. For instance, the true relationship between the introduction of farming and of chambered cairn construction is poorly documented. The earliest dates from Scottish chambered tombs are from bulk samples which for the most part relate neither to use nor construction but to activity on underlying old ground surfaces as at Port Charlotte (Harrington and Pierpoint 1980; RCAHMS 1984:50– 2) and Camster (Masters 1997:133, 157) or, as at Monamore, are arguably either less precise than claimed or misinterpreted (MacKie 1966a, 1966b). Although an age from Tulloch of Assery B for animal bones on the floor of the tomb (Sharples 1986) may hint that the commonly accepted picture of construction and use of chambered tombs starting around 4000 BC, or at least significantly earlier than 3500 BC, is not far wrong, it may equally well be that chambered tombs were first built in Scot- land at a significantly later date than the introduction of farming. Analysis of the 14C ages in the list will also probably help to generate new models. For instance, there is an intriguing hint in the distribution of 14C ages that the use of cinerary urns and related domestic urns may neatly fill the period between primary use of late Neolithic ceremonial sites and their re-use in the later Bronze Age (Ashmore, forthcoming). In other words, the ceremonial sites may have fallen out of memory for over half a millennium before they became one of the preferred sites for burial. It will also help the testing of competing ideas. For instance, the list makes it immediately apparent that more secure dating of the Stones of Stenness (Ritchie 1976) and Knap of Database for Scottish Archaeological Samples 45

Howar (Ritchie 1983) has the potential to help to resolve current controversies over social models for early Orkney. More generally, the opportunities for improvement provided by reduction in the size of samples required, and improvements in precision, are highly significant. Publication of Scottish archaeological ages in a form that allows an overview means that gaps in the dating of structural and pottery types can be better identified and addressed. Taking these factors together, the future for modeling of Scotland’s heritage is indeed rosy. Almost all of the age measurements commissioned by Historic Scotland before the end of May 1995 were carried out by the two Scottish laboratories whose laboratory codes have the prefixes GU and SRR. The original Scottish Universities Research and Reactor Centre (SURRC) Radiocarbon Dat- ing Laboratory (SRR laboratory code) was established by Doug Harkness in East Kilbride. The laboratory produced many of the early Scottish archaeological age measurements, however, in 1974, it moved to an adjacent building on site when its remit changed and it became a central facility for the UK’s Natural Environment Research Council (NERC), providing dating support in the environmental sciences. The Glasgow University Laboratory (GU laboratory code), which was run by Mike Stenhouse and subsequently Gordon Cook, continued to carry out Scottish archaeological work funded by the SDD Ancient Monuments Division (subsequently Historic Scotland) as well as its own in-house research. In 1986 the Glasgow University Radiocarbon Laboratory was relocated at SURRC (now SUERC) under the direction of Gordon Cook. Thus, the two Scottish 14C laboratories were then, and are still, in adjacent buildings; one carrying out dating support for NERC and the other having a largely archaeological remit, both carrying out their own in-house research. This proximity has been mutually beneficial and the two laboratories have secured joint funding for several research projects. It is therefore a logical extension of this collaboration that the laboratories should jointly publish all of their archaeological measurements in a database format in conjunction with Historic Scotland.

Quoted Errors and User Confidence The concept of error terms associated with age measurements has changed over the past two decades. The accuracy of 14C dating has increased considerably, and several international studies initiated by Dr Marian Scott and the Scottish laboratories (International Study Group 1982; Scott et al. 1990, 1997; Gulliksen and Scott 1995) and others (Otlet et al. 1980; Rozanski et al. 1992) have encouraged 14C dating laboratories worldwide to ensure that the results they produce are accurate and that their quoted errors accurately reflect their true analytical confidence. The conclusion of the International Study Group (1982) was that, while the results from the laboratories included in the study were in general agreement, they revealed the existence of systematic bias and unexplained variability. As a general guideline for users of 14C dates, it was advised that quoted errors, particularly those derived solely on the basis of counting statistics, should be multiplied by a factor of between 2 and 3. Dr M Stenhouse of the Glasgow University Laboratory recommended in 1982, on the basis of his participation in the study, that all the errors he had quoted for laboratory numbers up to GU-1500 should be multiplied by 1.4 and that these age measurements should be subject to a minimum error of ± 110 yr after multiplication (MJ Stenhouse, personal communication.). Intercalibra- tion studies confirm that since the mid 1980s, the quoted errors of the GU laboratory which has provided the majority of archaeological ages have in general been representative of the true errors and that there has been no significant bias in its results. The following table indicates the performance of the Glasgow University and NERC Radiocarbon Dating Laboratories in the Third International 46 P J Ashmore et al.

Radiocarbon Intercomparison (TIRI). The table demonstrates that both sets of results are, as a group, very much in line with the consensus results.

Table 1 TIRI performance of the Glasgow University and NERC 14C dating laboratories TIRI TIRI consensus result SURRC result NERC result sample code (pMCa or age BP ± 1σ) (pMC or age BP ± 1σ) (pMC or age BP ± 1σ) A 116.35 ± 0.0084 pMC 116.32 ± 0.58 pMC 115.42 ± 0.60 pMC B 4503 ± 6 BP0 4540 ± 50 BP 4,500 ± 45 BP C 129.7 ± 0.08 pMC 129.34 ± 0.54 pMC 130.21 ± 0.64 pMC D 3810 ± 7 BP0 3830 ± 50 BP 3780 ± 45 BP E 11,129 ± 12 BP0 11,270 ± 70 BP 11,090 ± 50 BP F 46,750 ± 208 BP >43,000 BP >62,000 BP G 39,794 ± 620 BP 40,300 ± 2520 BP >51,700 BP H 11,152 ± 23 BP0 11,150 ± 70 BP 11,180 ± 40 BP I 11,060 ± 17 BP0 11,040 ± 60 BP 11,135 ± 40 BP J 1605 ± 8 BP0 1590 ± 50 BP 1635 ± 40 BP K 18,155 ± 34 BP0 17,900 ± 140 BP 18,135 ± 100 BP apMC = percent modern carbon

It is well known that in the past, some laboratories often quoted mainly or only the “counting errors” associated with the sample, background and modern reference standard activities. They did not include some or all of the several other possible sources of error in their measurements. Some of the additional sources of error, associated with these early assays, may have been systematic while others may have been random, but larger than allowed for by the laboratory in quoting the error on the measurement. It seems very likely indeed that the errors attached to the 14C ages produced by many laboratories (but not all) up to the early to mid 1980s should be increased significantly if they are to be comparable with errors quoted today. Although it may be invidious to single out one laboratory, comparison of GaK ages for sites in the Pacific with those obtained by other laboratories has suggested that the errors attached to GaK dates are very considerably higher than those quoted by the laboratory (Spriggs and Anderson 1993). In the described database, some more realistic errors are indicated in the next-to-last column (although it must again be emphasized that these are open to challenge, because there is no scientific basis for attaching any particular correction factor to any particular age measurement in this list (apart from those GU ages covered by Dr Stenhouse’s advice) and because there is no widespread agreement on how to correct early error assessment. In general, the error terms attached to more recent 14C age measurements are reliable. The calibrated ages are based on these more realistic errors because it seems most sensible to apply the precautionary principle: better to be occasionally overcautious than to run the risk of spurious precision. The 14C ages were calibrated using OxCal 2.18 (Bronk Ramsey 1995) and the 1998 calibration curve (Stuiver et al. 1998). Dates marked forthcoming should not be quoted without the consent of the person named in the database. Database for Scottish Archaeological Samples 47

REFERENCES Ashmore PJ. 1996. A list of Historic Scotland archaeo- Masters L. 1997. The excavation and restoration of the logical radiocarbon dates. In: Turner R, editor. Discov- Camster long chambered cairn, Caithness, Highland. ery and excavation in Scotland. Edinburgh: Council Proceedings of the Society of Antiquaries of Scotland for Scottish Archaeology Publishers. p 136–42. 127:123–83. Ashmore PJ. 1997. A list of Historic Scotland archaeo- Olson EA, Broecker WS. 1959. Lamont natural radiocar- logical radiocarbon dates. In: Turner R, editor. Discov- bon measurements V. American Journal of Science ery and excavation in Scotland. Edinburgh: Council Radiocarbon Supplement 1:1–28. for Scottish Archaeology Publishers. p 112–7. Otlet RL, Walker AJ, Hewson AD, Burleigh R. 1980. 14C Ashmore PJ. 1998. A list of Historic Scotland archaeo- interlaboratory comparison in the UK: Experiment de- logical radiocarbon dates. In: Turner R, editor. Discov- sign, preparation and preliminary results. Radiocar- ery and excavation in Scotland. Edinburgh: Council bon 22(3):936–46. for Scottish Archaeology Publishers. p 125–8. Piggott S. 1954. The Neolithic cultures of the British Ashmore PJ. 1999. Radiocarbon dating: avoiding errors Isles. Cambridge: Cambridge University Press. in dating by avoiding mixed samples. Antiquity 73: Royal Commission on the Ancient and Historical Monu- 124–30. ments of Scotland. 1984. Argyll: an Inventory of the Ashmore PJ. Settlement in the second millennium BC in Monuments, 5: Islay, Jura, Colonsay and Oronsay. Scotland. In: Bruck J, editor. Landscape and settle- Edinburgh: HMSO Publishers. ment in Bronze Age Britain. Oxford: Oxbow Publish- Renfrew AC. 1989. Investigations in Orkney. Rep. Re- ers. Forthcoming. search Comm. Soc. Ants. London. London. Barcham RC. 1980. A lost radiocarbon date for Shetland Renfrew AC, Harkness DD, Switsur VR. 1976. Quanter- Islands. Proceedings of the Society of Antiquaries of ness, radiocarbon and the Orkney cairns. Antiquity 50: Scotland 110:502–6. 194–204. Bronk Ramsey C. 1995. Radiocarbon calibration and Ritchie A. 1983. Excavation of a Neolithic farmstead at analysis of stratigraphy: the Oxcal program. Radio- Knap of Howar, Papa Westray, Orkney. Proceedings carbon 37(2):425–30. of the Society of Antiquaries of Scotland 113:40–121. Buck CE, Cavanagh WG, Litton CD. 1996. The Bayesian Ritchie JNG. 1976. The Stones of Stenness, Orkney. Pro- approach to interpreting archaeological data. Chich- ceedings of the Society of Antiquaries of Scotland 107: ester: Wiley Publishers. 1–60. Calder CST. 1952. Report on the Excavation of a Rozanski K, Stichler W, Gonfiantini R, Scott EM, Beu- Neolithic Temple at Staneydale in the Parish of Sand- kens RP, Kromer B, Van der Plicht J. 1992. The IAEA sting, Shetland. Proceedings of the Society of Anti- 14C intercomparison exercise, 1990. Radiocarbon quaries of Scotland 85:185–205. 34(3):506–19. Edwards AWF. 1992. Likelihood: expanded edition. 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Radiocarbon dates from three cham- rings. Nature 198:619–23. bered tombs at Loch Calder, Caithness. Scottish Ar- Kra R. 1988. The first American Workshop on the inter- chaeological Review 4(1):2–10. national radiocarbon data base. Radiocarbon 30(2): Sheridan JA. 1997. The National Museums of Scotland 259–60. archaeological radiocarbon dating programme: dates Kra R. 1989. The international radiocarbon data base: a obtained during 1997. In: Turner R, editor. Discovery progress report. Radiocarbon 31(3):1067–74. and excavation in Scotland. Edinburgh: Council for MacKie EW. 1963–1964. New excavations on the Scottish Archaeology Publishers. p 117. Monamore Neolithic chambered cairn, Isle of Arran. Simpson DDA. 1996. Excavation of a kerbed funerary Proceedings of the Society of Antiquaries of Scotland monument at Stoneyfield, Raigmore, Inverness, High- 97:1–34. land, 1972–3. Proceedings of the Society of Antiquar- MacKie EW. 1964. The radiocarbon dates from a ies of Scotland 126:53–86. 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Polynesia. Antiquity 67:200–17. Walker AJ, Kra R. 1988. Report on the International Ra- Stuiver M, Reimer PJ, Bard E, Beck JW, Burr GS, diocarbon Data Base (IRDB) Workshop, Archaeology Hughen KA, Kromer B, McCormac G, Van der Plicht and 14C Conference, Groningen, The Netherlands. Ra- J, Spurk M. 1998 INTCAL98 radiocarbon age calibra- diocarbon 30(2):255–8. tion, 24,000–0 cal BP. Radiocarbon 40(3):1041–84. RADIOCARBON, Vol 42, Nr 1, 2000, p 49–52 © 2000 by the Arizona Board of Regents on behalf of the University of Arizona

FURTHER TESTS OF THE EDTA TREATMENT OF BONES

Ingrid U Olsson Department of Physics, Uppsala University, Box 530, SE-751 21 Uppsala, Sweden. Email: [email protected].

ABSTRACT. A new suite of five dates on a whale rib from Varangerfjord was completed on different fractions obtained by EDTA treatment. The intention was to test the possible influence of contaminants, the criteria for complete reactions, and the reliability of the treatment in light of scattered values obtained earlier on samples from Varangerfjord. The yield on the treatment of the selected bone did, however, not allow any general conclusions regarding the influence of contaminants in nature. The results are interesting from an inter-sample comparison point of view. Included are observations, made during treatment, of pH and color changes as well as the appearance of the samples. These observations are provided as a reference for deciding when the treatment is complete.

INTRODUCTION Berger et al. (1964) mentioned the possibility of using EDTA for bone treatment. They, however, suggested a dialysis bag, so the treatment seems less reliable than similar treatments in Groningen (Vogel and Waterbolk 1967:113; Grootes 1968) and in Uppsala. The EDTA1 method has been used in Uppsala since in 1964 and the first results were published 3 yr later (Olsson et al. 1967). The method was suggested in 1962 by Hendrik de Waard in Groningen, where he performed an EDTA treatment in the same year. The details of the method were given, and the reliability was examined in the laboratory (Olsson et al. 1974; El-Daoushy et al. 1978) using several bone samples, mostly from whales and seals from the Arctic region. The original method was further developed in Uppsala with recom- mendations that the material should be ground and that long extraction times be used to allow the liquids to penetrate into the inner parts. The extractions, each lasting normally two days or longer, should be repeated several times until no changes were observed for two or more extractions. This meant more than half a dozen extractions were needed. Special attention was also paid to the removal of EDTA from the sample because of the organic carbon. Studies of pH, color, and appearance changes were used as criteria for the completion of the treatments. It was also found that the final step, introduced in Uppsala, was very important. At this step, the sample was divided into a “right” fraction (R) and a “wrong” fraction (W), after the removal of any EDTA by adding dilute (0.1-N) HCl, evaporation and a final dissolution in hot water to yield the “right” fraction, whereas the insoluble part was rejected as the “wrong” fraction. Many examples were given of samples yielding a wrong fraction with too young a date. At the same time, various HCl treatments were tested. The EDTA method proved to be the preferred one. It should, however, be remembered that the contamination may differ from sample to sample. Unsuccessful results can be used to discredit a fraction obtained when applying a certain method. A successful result does not allow the conclusion that a specific method always is good. When several people have treated the same sample by applying a certain method, possibly slightly differently or using various methods, and have obtained the same date within the limits of uncertainty, the probability increases that the method is reliable.

EXPERIMENTAL TECHNIQUES

The Sample, its Treatment, Observations, and Activity Measurement

Earlier Dates from Varangerfjord The sample, a whale rib, was collected at Varangerfjord, Finnmark, northern Norway, and submitted by Joakim Donner. It was originally used for a comparison with a shell sample, Mytilus edulis, dated

1EDTA is the sodium salt of ethylenediamine tetraacetic acid.

49 50 I U Olsson by the Helsinki Radiocarbon Laboratory. The shell sample was regarded as a reliable sample (Don- ner et al. 1977) in contrast to a Mya truncata sample. The δ13C value was given as +1.7‰ and the original date, without any normalization for the δ13C value, as Hel-624: 4120 ± 130 BP. The normalized age to be used is thus 4560 ± 140 BP. Five dates were presented by El-Daoushy et al. (1978). Two of them were determined on the same gas, after HCl treatment, but using two different proportional counters. Two of them were measured after the same EDTA treatment but one on the gas obtained at the degassing and the other at the real combustion. The ages as determined on gas from the HCl treatments, including usage of the iso-electric point, were significantly younger than those after the EDTA treatment. One δ13C value was significantly different from the other three values. The Helsinki age value was almost 2σ younger than the mean of the two EDTA values from Upp- sala. The significant spread implied that the whale rib might be suitable for further studies.

Pretreatment and Summary of the EDTA Treatment The sample was partly covered by lichen. The outer layer was removed by scraping and sawing. The sample was washed twice in distilled water in an ultrasonic bath. It was dried and ground. As a result of this pretreatment the sample was reduced from 330 to 260 g before commencing the EDTA treatment. The main yield was three “right” fractions, each equivalent to about 2 g carbon. Each of these fractions was dated. The “wrong” fractions were much lighter in carbon but each was split into two halves of which one was further treated with successively stronger HCl to yield samples soluble in 1% HCl, 1-N HCl, and 4-N HCl. The yield was so small that it seemed meaningless to date them in order to trace the contaminant. Two samples were, however, dated, although after dilution with inac- tive carbon dioxide to bring the gas pressure in the counter to a normal level. With a dilution factor higher than 10 the statistical uncertainty increased considerably. This was to some extent reduced by repeated measurements.

Possible Explanation of an Error to be Examined Because of the younger ages obtained at the present investigation, further details regarding the treatments must be given to explain the differences.

Observations of the EDTA Treatment (in 1979) The treatment with EDTA and the washings were performed in a beaker. The separation of the sample and liquid was possible by filtering, at the first treatment, and then by decanting and centrifuging in later steps. After four EDTA treatments, a quarter of the sample seemed gelatinous, after five treatments half of it, and after six, all of it was gelatinous. The liquid turned dark brown almost at once in the first treatment but was light brownish after the fourth, and light yellow-brownish after the sixth. No pH difference between the used EDTA and the prepared solution could be shown after three EDTA treatments. One-third of the sample was taken after three treatments for water washing and then half of the remaining sample after three further treatments. Thus there were three samples: 3 EDTA R, 6 EDTA R, and 9 EDTA R, where R stands for the “right fraction”. The total time spent on the EDTA treatment was 26 days. The corresponding three “wrong fractions” were further treated as described above.

Removal of EDTA The water wash, six washes for each of the right fractions, lasted in total 18, 14, and 17 days, respectively. The separation was performed by decanting and/or centrifuging. After two to four washes, the pH of the water seemed unaffected; it turned opalescent and was virtually colorless. Tests of the EDTA Treatment 51

Notes on the Previous EDTA Treatment (in 1975) The treatment in 1975 was, however, made in a glass filter placed in a beaker. No notes on vacuum suction were found. The EDTA treatment lasted for 29 days and was performed in eight steps. After the sixth step, the pH was unaffected, and after the seventh time it was noted that the sample was soft. The sample was washed six times, in total for 27 days, the pH was affected in the first two washes, and for the last two washes the water appeared colorless. The explanation of the age difference is presently based on an argument of insufficient removal of the EDTA solution, since EDTA is synthesized from old carbon.

Measurements The activity measurements were performed in 1979 and 1980 using carbon dioxide in the same counter (PR5) as used for four of the Varanger results in the 1978 publication. The sample values and the standard values for background and oxalic acid were re-evaluated in 1999. It was found that the statistics for each sample (background, oxalic acid, and the five samples) were very good. Although the measurements were spread over more than a year, the electronics were adjusted during this period, so the measurements must be regarded as independent of each other. Indeed, three values for each of the background and oxalic acid had to be used. The δ13C values were determined in Stockholm by R Ryhage and his coworkers.

Table 1 All dating results after different treatments of a whale rib from Varangerfjorda δ13C ‰ 14C age Lab nr (PDB) (yr BP) Treatment and fraction Year treated, comments U-4128 −16.7 4095 ± 100 Soluble in HCl, iso-electric point, centrifuged, 1975 dissolved in HCl, centrifuged, dialyzed U-4127 − 4320 ± 320 Insoluble in HCl, dissolved by heating, iso- 1975, same gas mea- U-2751 12.5 4085 ± 190 electric point, centrifuged, dialyzed, dissolved sured in 2 different in HCl, dialyzed counters U-4125 −17.2 5140 ± 170 1975, degassing EDTA (soluble in H O - Right f) U-4126 −16.6 4710 ± 150 2 1975, real combustion U-4331 −14.61 4530 ± 120 EDTA, 3 times, (3 EDTA R) 1979 U-4332 −15.95 3960 ± 220 EDTA, 6 times, (6 EDTA R) 1979 U-4333 −15.00 4310 ± 090 EDTA, 9 times, (9 EDTA R) 1979 Insoluble in 0.1-N HCl after 3 EDTA, soluble 1979, yield < one U-4334 −17.26 3760 ± 350 in 1% HCl tenth of 3 EDTA R Insoluble in 1% HCl after 3 EDTA, soluble in 1979, yield << one U-4335 −17.10 4830 +1080 −940 1-N HCl tenth of 3 EDTA R aResults are assumed to have the same age as a Mytilus edulis sample dated in Helsinki at 4560 ± 140 BP, after normalization to δ13C = −25‰.

RESULTS The results of the new investigation are given in Table 1 together with the previous results. The spread of the dates seems to be due to differences between the gaseous samples. The spread of the δ13C values also indicates a possible difference. The large σ-values for the two samples deriving from the “wrong fraction” are due to dilution. Similarly, part of the “right fraction” after six EDTA treatments was lost, so the statistical uncertainty is rather high. A small error in the background value will increase the uncertainty in the final normalized activity values for diluted samples. The 52 I U Olsson difference between the oldest and youngest age values for the “right fractions”, three EDTA R and six EDTA R, is slightly larger than 2σ. Despite this, the spread is not larger than can be expected to sometimes occur. Τhe three new age values for “right fractions” are significantly younger than the mean age obtained in 1976 using the EDTA treatment and published in 1978. The two values obtained for fractions extracted from what normally is the “wrong fraction” (this time after three EDTA treatments) are not significantly different from the three “right fraction” dates. The HCl treatment in 1975 yielded fractions with not significantly different ages.

DISCUSSION AND CONCLUSIONS The separation of the sample from the used EDTA solution or the water at the washings may have been much more efficient in 1979 than in 1975 because of the centrifuging. The importance of this step was already stressed in the papers from 1974 and 1978. The conclusion is that EDTA treatment can yield reliable ages, when carefully applied. The sample should be ground. The EDTA extractions are not laborious but the treatment should be repeated several times and extended over long periods. Washing with water is very important and care should be taken to separate the sample from the liquid after each washing. Besides centrifuging, suction through a glass filter can be used. No severe contamination in nature could be detected at this investigation. The sample did not prove suitable for testing whether the EDTA method is reliable for any bone sample, but did prove suitable for testing the slow reaction in each step of the treatment. The yield of the “wrong fractions” was too small to significantly affect the final results. Indeed these fractions were not evaporated down and then dissolved in water. The strong HCl dissolved material was burned after evaporation and should thus not be called “right fractions”. For an extended investigation of the EDTA method, tests using a weaker HCl than 0.1-N is suggested.

ACKNOWLEDGMENTS The author is very thankful for the continuous support from the Swedish Natural Science Research Council during her employment in the Laboratory. I would like to thank Hans Nyhlén for being responsible for the present treatment together with me and the technicians, mainly one assisting in 1975 and another in 1979, and those assisting at the activity measurements. I will also address my thanks to the head of the Department allowing me to write this summary of old measurements.

REFERENCES Berger R, Horney AG, Libby WF. 1964. Radiocarbon Grootes PM. 1968. Bereiding van collagen uit botten dating of bone and shell from their organic compo- voor dateringsdoeleiden. Thesis, Groningen. 69 p. nents. Science 144:999–1001. Olsson IU, El-Daoushy MFAF, Abd-El-Mageed AI, Donner J, Eronen M, Jungner H. 1977. The dating of the Klasson M. 1974. A comparison of different methods Holocene relative sea-level changes in Finnmark, for pretreatment of bones. I. Geologiska Föreningens North Norway. Norsk Geografisk Tidsskrift 31:103– i Stockholm Förhandlingar 96:171–81. 28. Olsson IU, Stenberg A, Göksu Y. 1967. Uppsala natural El-Daoushy MFAF, Olsson IU, Oro FH. 1978. The radiocarbon measurements VII. Radiocarbon 9:454– EDTA and HCl methods of pre-treating bones. Geol- 70. ogiska Föreningens i Stockholm Förhandlingar 100: Vogel JC, Waterbolk HT. 1967. Groningen radiocarbon 213–9. dates VII. Radiocarbon 9:107–55. RADIOCARBON, Vol 42, Nr 1, 2000, p 53–68 © 2000 by the Arizona Board of Regents on behalf of the University of Arizona

RADIOCARBON DATING THE LAST GLACIAL-INTERGLACIAL TRANSITION (Ca. 14–9 14C ka BP) IN TERRESTRIAL AND MARINE RECORDS: THE NEED FOR NEW QUALITY ASSURANCE PROTOCOLS1

J John Lowe Centre for Quaternary Research, Department of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom. Email: [email protected]. Michael J C Walker Department of Geography, University of Wales, Lampeter, Ceredigion, Wales, SA48 7ED United Kingdom

ABSTRACT. The publication during the 1990s of Greenland ice-core records spanning the transition from the Last Cold Stage to the present interglacial (ca. 14–9 14C ka BP) presented new challenges to scientists working on marine and terrestrial sequences from this important time interval. In particular, there is now an overriding imperative to increase the levels of precision by which events during this period can be dated and correlated. We review some of the problems commonly encountered when using radiocarbon dating for these purposes, and consider some of the new approaches that will be required if this dating method is to provide a basis for a high precision chronology for the last glacial-interglacial transition.

INTRODUCTION Between approximately 14.0 and 9.0 14C ka BP the last glacial cycle drew to a close in a series of abrupt and apparently pronounced climatic oscillations, and was finally terminated by a sustained shift to warmer conditions at the start of the present (Holocene) interglacial. This transitional period has been referred to, inter alia, as the “last glacial-interglacial transition” (LGIT), the “last glacial- Holocene transition”, the “Late-glacial period” and the “Last Termination”. For simplification, we will use LGIT to denote this period of time. Because the LGIT is the most recent example in the geological record of a transition between a full glacial and an interglacial climatic régime, and because the geological evidence available for the reconstruction of events during this period is unsurpassed in terms of its level of preservation and diversity, it is one of the most intensively studied intervals in the entire Quaternary record. As a consequence, paleoenvironmental reconstructions for this period are some of the most highly resolved, in terms of their temporal and spatial detail, of any part of the Quaternary sequence (e.g. Lowe et al. 1994; Lundqvist et al. 1995; Lowe and Walker 1997a, Rens- sen and Isarin 1998; Walker et al. forthcoming). The geochronological underpinning of these reconstructions, in terms of the development of site chronologies, regional syntheses and inter-regional correlations, has relied almost exclusively on 14C dating. Indeed, it is difficult to over-estimate the importance of the role that 14C has played in the development of the LGIT research agenda over the course of the last four decades. For example, the chronostratigraphic framework that has, hitherto, been most widely employed in NW Europe for the subdivision of the LGIT was defined exclusively in 14C years (Mangerud et al. 1974). This scheme consisted of four principal elements: the “Bølling Chronozone” (including the “Oldest Dryas” episode), defined as the time interval between 13.0 and 12.0 14C ka BP, the “Older Dryas Chronozone” (12.0 to 11.8 14C ka BP), the “Allerød Chronozone” (11.8 to 11.0 14C ka BP) and the “Younger Dryas Chronozone” (11.0 to 10.0 14C ka BP). Each of these was considered to coincide with major climatic events or episodes. The widespread application of the Mangerud et al. chronostratigraphic scheme and its associated terminology, even in regions or areas for which it was never originally intended (see Wohlfarth

1A contribution to the INTIMATE Programme of the INQUA Paleoclimate Commission

53 54 J J Lowe, M J C Walker

1996; Björck et al. 1998), reflected the widespread confidence of the scientific community in the 14C dating method. It was generally assumed that: a) the 14C technique provided a basis for dating and correlating climatostratigraphic boundaries with a reasonable degree of precision, and b) there was close correspondence between the 14C ages of the chronozone boundaries and the timing of climate change, at least in those regions where the scheme had been adopted. However, during the course of the last decade or so, questions have arisen about both of these assumptions. First, it has become increasingly apparent that, because of natural variations in the rate of 14C production, 14C years may have diverged from sidereal years by as much as 2.5 ka or more during parts of the LGIT (Bard et al. 1993). Superimposed on these long-term fluctuations have been shorter episodes of near-“constant age”, the so-called 14C “plateaux” (see e.g. Ammann and Lotter 1989; Bard and Broecker 1992; Lotter et al. 1992; Kromer and Becker 1992; Bard 1998; Stuiver et al. 1998). The fact that at least two of the chronozone boundaries in the Mangerud et al. (1974) chronostratigraphy fall within such 14C plateaux renders the scheme much less precise as a tool for dating and correlation than was appreciated only a quarter of a century ago (Walker et al. 1999a). Secondly, the climatic changes themselves, which are frequently used to define the boundaries of the “Bølling”, “Allerød”, and “Younger Dryas” intervals in regional stratigraphic schemes, appear to have been diachronous across northwestern Europe (Coope et al. 1998; Witte et al. 1998). This, of course, calls into question the assumption that stratigraphic boundaries defined on the basis of climatic change are synchronous over large areas. Measuring the extent of diachroneity, however, is not easy when employing conventional approaches to 14C dating, again because of the limits on the levels of precision that are achievable. Thirdly, although it was subsequently acknowledged that the sequence of climatic changes during the LGIT may have been more complex than was initially embodied in the Mangerud et al. zonation scheme (see e.g. Walker 1995; Yu and Eicher 1998), neither the degree of climatic variability nor the abruptness of the climatic transitions was fully appreciated until the publication of the GRIP and GISP2 ice-core data (e.g Alley et al. 1993; Dansgaard et al. 1993; Kapsner et al. 1995; Taylor et al. 1997; Alley 2000). These remarkable records are characterized by climatic events with durations as short as 150 ice-core years, and significant climate shifts measurable to within ice-core decades, or even ice-core years. Such temporal resolution appears to be well beyond the levels of precision presently attainable using 14C dating. In the light of these new developments, and because of the demands that are increasingly being placed upon researchers to improve the levels of precision in the dating and correlation of LGIT events, new strategies for 14C dating need to be formulated. In this paper, we discuss the shortcom- ings of 14C dating as currently applied to LGIT records, and we suggest alternative approaches that might help resolve at least some of these difficulties.

SOURCES OF ERROR IN 14C DATES OBTAINED FROM LGIT SAMPLES Error sources in 14C measurements on samples of LGIT age fall into three categories: 1) site-specific geological problems that adversely affect sample integrity, 2) laboratory contamination and measurement precision, and 3) calibration of 14C dates to sidereal years. Each of these is considered in this section.

Sample Selection and Integrity A wide range of materials is now employed in the dating of LGIT events, including fossil wood, terrestrial leaves and seeds, marine macro- (e.g. bivalves) and microfossils (e.g. foraminifera), fossil bone and bulk organic debris (e.g. “gyttja”, organic lake muds, and peats). Each has been affected to a greater or lesser extent by physical processes (including reburial or redeposition) and/or chemical Dating the Last Glacial-Interglacial Transition 55 alteration, depending on the geological context through which the fossil material was transported and/or into which it was eventually deposited. These “site-specific” factors may influence the integrity of the samples and thereby adversely affect the resulting 14C ages. A considerable amount of recent research has been devoted to the identification and quantification of, and subsequent correction for, these site-specific factors. Indeed, it could be argued that these essentially “taphonomic problems” are the most important that need to be resolved, since attempts to correct for other potential influences (e.g. laboratory contamination and temporal variations in atmospheric 14C production) can only result in spurious levels of precision if the geological integrity of the samples remains in question. The range of potential site-specific geological problems that can influence the suitability of samples for 14C dating is too diverse and complex to review in this short paper. Instead, we will focus on the problems that are particularly acute in the dating of LGIT sequences by examining those materials that are most commonly employed in the 14C dating of this time interval.

Bulk Organic Sediment Samples A large number of published chronologies for the LGIT from sites in Europe and North America are based on 14C dating series obtained from bulk organic lake or pond muds (“gyttja”). It has long been recognized that a number of potential error sources can affect bulk samples of these sediments, including, for example, hard-water and mineral carbon errors, and biological and chemical fractionation processes (see e.g. Lowe 1991; Wohlfarth et al. 1993; Wohlfarth 1996; Lowe and Walker 1997). In some instances, it is possible to reduce the adverse effects of these site- or sample-specific factors through careful site and/or sample selection procedures and/or by the application of correction factors. However, identifying the sites and samples least affected by these problems, and establishing the scale of corrections to apply are far from straightforward. Lake or pond sediments are composed of a heterogeneous mix of clastic particles and organic debris, the latter derived from biota either inhab- iting the lake or pond waters or from vegetation growing around the basin catchments (e.g. Colman et al. 1996; Batterbee 2000). The organic debris will, therefore, frequently include older organic materials eroded from the exposed lake edges (particularly during times of reduced water level) or from other organic deposits located in the catchment (e.g. exposed soils) which, often intermittently, may be influenced by fluvial, colluvial or other erosional processes. Thus bulk sediment samples consist of a variety of components which, if differentiated, would almost certainly generate a range of ages. The published 14C dates from the vast majority of bulk sediment samples are, therefore, almost certainly derived from averages of a range of activity values. 14C dates based on organic detrital limnic sediments obtained from LGIT sequences are often characterized by an “ageing effect” due to the dilution of the 14C:12C ratio in organic residues. This problem is particularly acute where the local bedrock (or surficial material such as glacial till) is calcar- eous, or where mineral particles rich in carbon which is both refractory and geologically old have become incorporated into the lake deposits. Sediments accumulating in lakes in northern latitudes during the LGIT were particularly susceptible to “ageing” effects because of the glacier melt processes and widespread land instability (e.g. gelifluction or solifluction processes) prevalent at the time. Soils containing both older organic and inorganic materials would have been eroded from catchment slopes, while “glacial flour”, which often contains carbonaceous residues, would also have been washed into many lake basins (e.g. Björck and Moller 1987; Andrieu et al. 1993; Walker et al. 1993; Lowe et al. 1995). The problem is further exacerbated by the “hard water factor” where sub-aquatic photosynthesizing plants which take up carbonate from lake waters will further dilute 56 J J Lowe, M J C Walker

14C levels in organic lake muds, in exceptional circumstances adding up to 1200 yr to the apparent age of limnic material (Peglar et al. 1989). That an “ageing” problem is commonly encountered in organic limnic sediments of LGIT age has been clearly demonstrated when 14C dates have been obtained on humic (acid washed, alkali soluble) and humin (acid washed, alkali insoluble) fractions of the same sediment sample. Older carbon residues, which would induce an ageing effect in a single bulk sediment date, tend to be reflected in the “humin” age determinations which may be older than the “humic” date by hundreds or, in some cases, thousands of years (Walker and Harkness 1990). Similar differences in ages have been noted between AMS 14C measurements on separate macromolecular compounds (e.g. lipids, amino acids and cellulose) extracted from the same bulk samples (e.g. Lowe et al. 1988). Equally, where parallel 14C measurements have been obtained on both the sediments and on plant macrofossils, the age estimates on the macrofossils are frequently younger than those obtained from their host sediment matrix (e.g. Coope and Brophy 1972; Coope and Joachim 1980; Cwynar and Watts 1989). In recent years, therefore, there has been a tendency to assume that the 14C dating of plant macrofossils offers a more reliable means of establishing a chronology for the LGIT, since this avoids many of the difficulties encountered in the dating of bulk sediment samples (see e.g. Peteet et al. 1995; Lowe et al. 1995a; Björck et al. 1996; Gulliksen et al. 1998; Hughen et al. 1998; Kitagawa and Van der Plicht 1998). However, this is an assumption which has seldom been tested rigorously and, as is shown in the following section, it may not always hold true.

Dating Plant Macrofossils With the rapid development of, and expansion in, AMS facilities over the last decade, plant macrofossils are now being used as a routine material for 14C dating of LGIT sequences since a reasonable analytical precision can be obtained from as little as 2 mg of organic carbon. This amount of organic C can typically be extracted from just a few fossil seeds, fruits or bracts, or even from a single fossil leaf. There has been a tendency to avoid aquatic taxa (e.g. Potamogeton) in such dating practices as many aquatic plants photosynthesize subaquatically and hence build into their cellular material the 14C:12C ratios of the lake waters they inhabit, which may, in turn, be depleted by “hard-water” influences. However, a less critical approach has been taken towards the terrestrial plant macrofossils that have been dated, and it has generally been assumed that reliable AMS 14C ages can be obtained, irre- spective of the species of plant remains that are being dated. This assumption has recently been challenged, however. Turney et al. (2000) report AMS 14C dates from Salix herbacea leaves, Carex seeds and bulk organic detritus from a LGIT profile at Finglas River in southwest Ireland. These show systematic age differences between the dated series from the two types of macrofossils, with those obtained from Salix herbacea leaves being 900–1500 14C yr younger than those obtained from Carex seeds from the same horizons in the profile. The Carex results tend to be more in accord with a third series of dates obtained from samples of bulk organic detritus, although the latter invariably registered the oldest age in each dated horizon. Careful evaluation of the three dating series and of the litho- and biostratigraphic contexts suggested that the Salix leaves provided the most reliable age estimates, that the bulk organic detritus samples were probably contaminated by mineral carbon and/or older organic detritus reworked from the catchment soils which also incorporated the Carex seeds. Similarly, recent investigations at the site of St. Bees in northwest England show systematic differences in 14C age between terrestrial plant macrofossils of LGIT age and coleopteran remains (Walker et al. 1999b). Dating the Last Glacial-Interglacial Transition 57

These data suggest that certain types of plant macrofossil (e.g. Carex seeds in the case from Finglas River site) may provide age estimates that are as aberrant as those obtained from contemporaneous lake sediments in which mineral carbon or hard-water-errors may occur. The conclusion must therefore be that, in certain sites and under certain conditions, AMS dates on terrestrial plant macrofossils are no more superior to bulk sediment samples in the dating of events during the LGIT. Indeed, it may well be that the most reliable chronology can be obtained not from the plant macrofossils, but rather from the “humic” sediment component, i.e. that part of the sediment fraction where there will effectively be no contamination by older carbon residues. In terms of dating strategy, therefore careful biostratigraphical investigations are required, coupled with the careful screening of plant macrofossils prior to dating, especially where high-precision geochronology is the objective.

Marine Reservoir Effect Fossils obtained from marine sequences display an “apparent age”, or marine reservoir effect, caused by the slow mixing of ocean waters, and the upwelling of 14C-depleted waters near some coasts. Accordingly, 14C laboratories normally advise on an appropriate correction factor for those 14C measurements obtained from marine fossils, which is derived by measuring 14C activity in contemporary marine organisms. Examples of the scale of this marine reservoir correction are 190 ± 40 for coastal Peru and Chile (Southon et al. 1995), 355 ± 20 14C yr for coastal Iceland (Håkansson 1983), about 400 14C yr for coastal waters of the UK, parts of the North Atlantic and submerged corals around Barbados (Harkness 1983; Bard et al. 1987, 1991; Southon et al. 1992), about 580 14C yr for the Adriatic Sea and parts of the eastern Pacific (Shackleton et al. 1988; Langone et al. 1996) and 788 ± 33 14C yr for the northeastern Pacific (Southon et al. 1992). It is important that the values of marine reservoir correction factors are firmly established, in view of the fact that marine circulation played a key role in driving climate perturbations during the LGIT (Björck et al. 1996). Uncertainty over the appropriate correction factor to apply reduces the level of precision in correlations between marine and terrestrial LGIT sequences (e.g. Asioli et al. 1999) and between marine and ice-core records (e.g. Voelker et al. 1998). It also limits the precision with which the timing of global meltwater discharge and sea-level rise can be reconstructed (e.g. Bard et al. 1996). A more serious difficulty has recently emerged with respect to marine reservoir effects, namely that marine reservoir values may not only have varied spatially, but also temporally during the LGIT. Comparison of 14C ages obtained from marine planktonic foraminifera and from terrestrial deposits of the same age, age-equivalence having been established by tephrochronology (using the Vedde Ash Bed, Wastegård et al. 2000), suggest that the atmosphere-sea surface 14C difference in the North Atlantic was approximately 700–800 14C yr at the time of deposition of the Vedde Ash (during the “Younger Dryas/Greenland Stadial 1”) compared with the present difference of around 400–500 14C yr (Bard et al. 1994). By contrast, reservoir ages along the Norwegian coast during the preceding interstadial (“Bølling/Allerød/Greenland Insterstadial 1”) were comparable with present-day values (Bondevik et al. 1999). In the Baltic Sea, however a marine reservoir age in excess of 1000 yr has been inferred for the early Holocene (Björck et al. 2000). Indeed over the course of the last 50 ka, the magnitude of the reservoir effect in the Iceland and Norwegian Seas may have varied by as much as 1600 14C yr, perhaps as a result of variations in geomagnetic field intensity, coupled with ocean circulation changes (Voelker et al. 1998). Clearly, further work is needed in order to improve our understanding of temporal (and spatial) variations in the magnitude of marine reservoir effects in different ocean sectors. Comparison of 14C measurements obtained from marine and terrestrial fossils whose age-equivalence has been estab- 58 J J Lowe, M J C Walker lished by some independent means (e.g. tephrochronology or paleomagnetic stratigraphy) would appear to be the best way forward.

Analytical Precision and Laboratory Contamination Two problems that are frequently encountered in the dating of LGIT sediment sequences, and which affect the levels of analytical precision that can be attained, are: 1) the fact that most organic sediments that accumulated during the LGIT, especially in northern latitudes, have a low organic C content, and 2) the majority of investigations of this time period have been based on coring which inev- itably restricts the size of sediment samples that are available for dating. The latter applies equally to the AMS dating of plant macrofossils, as many LGIT sediments have relatively low fossil concentrations. In the case of radiometric dating, a combination of low organic C content and limited sediment sample size impacts adversely on analytical precision. As a consequence, the standard errors on LGIT samples are typically around, or even in excess of, 100 14C yr, even in the case of measurements completed relatively recently (e.g. Preece and Bridgland 1999). This level of precision is not markedly different from that achieved in the early days of 14C dating. However, where open section sites have been sampled, and commensurately larger volumes of material extracted for dating purposes, counting statistics have improved dramatically with levels of precision of ±45 to ±50 being routinely achieved (Walker et al. 1993, 1999b). Nevertheless, further improvement in precision would seem unlikely, unless unusually long counting times are employed (Pilcher 1991). In the case of AMS dating, the limits on precision are much more a function of the purity of the targets prepared and the performance capability of the AMS equipment (e.g. Van der Plicht 1995; Chen et al. 1995). It should be noted, however, that samples for AMS dating are often very small (less than 4 mg by weight and with a carbon yield measurable in µg), and often with a low organic C content. Although significant improvements have been made in both target preparation and in equipment sensitivity, the levels of precision of 14C dates achieved routinely by AMS measurement of samples of LGIT age remains between ±50 and ±150 (e.g. Sirocko et al. 1993; Björck et al. 1996; Gulliksen et al. 1998; Voelker et al. 1998; Preece and Bridgland 1999; Asioli et al. 1999; Rühlemann et al. 1999), even where weighted means from multiple measurements have been employed (Burr et al. 1998). Indeed, ±100 yr and above is frequently the norm, and it is one of the few disappointments of the AMS method that, despite initial aspirations to achieve higher levels of analytical precision, in many instances counting statistics have not improved significantly. Clearly, in view of the temporal resolution that is now required in the reconstruction of events during the LGIT, further improvement in levels of 14C dating precision remains a research priority. Although increased precision may be best achieved, perhaps, through “wiggle-matching” (to calibration curves) of a series of 14C dates (see below), there is, nevertheless, an imperative to improve analytical precision still further through improved sample preparation protocols, use of additional counters to enable longer counting times, more rigorous analysis of variability in laboratory standards, and the development of even purer blanks and targets (e.g. McNichol et al. 1995; Schneider et al. 1995; Bird et al. 1999). The general tendency towards the use of smaller samples, and/or samples of very low organic C content, in AMS 14C dating places even greater onus upon the 14C “user” community to take stringent precautions against contamination. The inadvertent introduction of miniscule amounts of modern organic C into samples of low 14C activity can have a major impact on the eventual date. Laboratory procedures should therefore be adopted which are designed to reduce the chances of even micro-lev- Dating the Last Glacial-Interglacial Transition 59 els of contamination to an absolute minimum, including, for example, ensuring that the final selection and washing of samples takes place in ultra-clean laboratories or within laminar-flow (positive pressure) cabinets, and avoiding contact with any organic solvents during the treatment of samples prior to despatch to the dating laboratory. The point is elaborated by Wohlfarth et al. (1998), whose work suggests that even the long-term storage of wet macrofossil samples in the laboratory can lead to contamination through the growth of fungi or micro-organisms.

Calibration For over two decades, a precise 14C-calibration curve based on dendrochronologically-dated samples has been available for much of the Holocene, but this has only recently been extended back to approximately 12,000 dendro-years BP (Kromer and Spurk 1998; Spurk et al. 1998), i.e. towards the close of the LGIT (12,000 dendro- or calendar years are equivalent to ca. 10,600 14C yr). Attempts have also been made to extend calibration further back into the LGIT interval, and beyond using inter alia, paired U-series and 14C dates on coral samples (Bard et al. 1998; Burr et al. 1998), 14C dating of fossils contained in, or bulk sediment samples obtained from, annually-laminated lake and marine sediments (Hughen et al. 1998a, 1998b; Kitigawa and Van der Plicht 1998a, 1998b), and synchronization or “tuning” of 14C-dated marine or terrestrial sequences to ice-core records (Voelker et al. 1998). It has been suggested that some of these data sets should be amalgamated to produce an integrated calibration curve for the period 12,000 to >45,000 yr ago (Jöris and Weninger 1996; Van Andel 1998), but Van der Plicht (1999) has argued that the various data-sets deviate from each other to such a degree (by several millennia at approximately 30,000 yr ago, for example) that little confidence can be attached to calibrations derived using such an approach. The calibration data-set most widely employed for samples dating beyond the dendro-based calibration curve is INTCAL98 (Stuiver and Van der Plicht 1998). The INTCAL98 data-set relies mainly on coral data (dated using both the U-series and 14C methods), but the paired age-points are few in number and many of the dates have relatively large statistical errors. Furthermore, a standard marine reservoir age of about 500 yr was adopted to link the measured 14C ages to the terrestrially defined 14C timescale, but this clearly fails to take account of the temporal variations in the magnitude of the marine reservoir effect (see above). As a consequence, using INTCAL98 to calibrate 14C dates from the LGIT often significantly increases the statistical uncertainty of the age estimates. The 14C “plateaux” (episodes of near-constant 14C age) that characterize parts of the LGIT time interval (e.g. Ammann and Lotter 1989; Lowe 1991; Bard and Broecker 1992; Lotter et al. 1992; Kromer and Becker 1992; Austin et al. 1995; Wohlfarth et al. 1993) could, in theory, provide a basis for improving the precision of 14C dates using a procedure analogous to the “wiggle-matching” approach. An elegant application of this method at a site in western Norway which employed 70 AMS 14C dates on both terrestrial plant macrofossils and NaOH-soluble fractions of lake sediment, “pin-pointed” the age of the Younger Dryas-Holocene transition to between 11,500 and 11,600 calendar yr BP (Gulliksen et al. 1998). Five clearly defined episodes of near-constant 14C age have been recognized during the LGIT centering on approximately 12.7–12.6, 11.4–11.3, 11.0–10.9, 10.4– 10.3, and 10–9.9 14C ka BP (see e.g. Kromer and Becker 1993; Goslar et al. 1995; Björck et al. 1996; Wohlfarth 1996). However, 14C plateaux are poorly resolved in the INTCAL98 data (Stuiver and Van der Plicht 1998) while in other calibration data sets, such as that based on the Cariaco Basin sequence (Hughen et al. 1998a), plateaux-like features occur over different intervals. At present, therefore, the 14C data from the LGIT do not permit the kind of precise “wiggle-matching” that the dendro-based calibration curve makes possible for the dating and correlation of Holocene events. 60 J J Lowe, M J C Walker

HIGH-PRECISION CORRELATION OF LGIT SUCCESSIONS: THE ROLE OF 14C DATING In recent years, remarkably similar paleoclimatic reconstructions for the LGIT have been derived from terrestrial fossil records, from marine sediment sequences, and from Greenland ice-core records. Fossil beetle data from the UK (Atkinson et al. 1987), oxygen isotope variations in lake sediments in Switzerland and Germany (Siegenthaler et al. 1984; Von Grafenstein et al. 1999), oxygen isotope ratios, dust content and snow accumulation records in Greenland ice cores (e.g. Alley et al. 1993; Dansgaard et al. 1993; Taylor et al. 1993), and variations in biological productivity in ocean floor sediments in the tropical Atlantic Ocean (Hughen et al. 1996) all show the same general and highly distinctive pattern (Figure 1), which is now so recognizable that it serves as a kind of leitmotif for the LGIT (see Lowe and Walker 1997b). Such a high degree of conformity between terrestrial, marine and ice-sheet records has prompted speculation that the major climatic changes during the LGIT were broadly synchronous, and that they were therefore orchestrated by a common forcing mechanism—possibly the North Atlantic ocean conveyor (Broecker et al. 1985). Many LGIT scenarios for the North Atlantic region are now based on this assumption, the marine or terrestrial records being “tuned” to the GRIP or GISP-2 ice-core records, as their highly resolved chronologies and wealth of paleoclimatic detail make them ideal templates for climatic reconstruction (e.g. Jöris and Weninger 1996; Hughen et al. 1998a; Voelker et al. 1998; Von Grafenstein et al. 1999). Indeed the INTIMATE group2 (Björck et al. 1998; Walker et al. 1999a) has recommended that the GRIP ice-core record be regarded as the stratotype for the LGIT in the North Atlantic region, and advocate an “event stratigraphic” approach to inter-regional correlation, one which is based on comparing regional paleoclimatic signals (“events”) with those recorded in the Greenland stratotype. The events are the pronounced, high-amplitude cold (stadial) and warmer (interstadial) episodes that are so clearly manifest in the GRIP isotope trace, but which can also be recognized in other proxy climate records from around the North Atlantic region (Figure 1).

Figure 1 Mean annual temperature variations in Britain during the LGIT (A) (from Atkinson et al. 1987) compared with (B) stable oxygen isotope variations in sediments in Switzerland (Siegenthaler et al. 1984), (C) gray-scale variations reflected in laminated sediments in the Cariaco Basin, tropical Atlantic (Hughen et al. 1996), and (D) oxygen isotope variations in the GRIP ice-core record (Dansgaard et al. 1993). The GRIP isotpoe stratigraphy for the FGIT is described in Walker et al. (1999).

2INTIMATE (INTegration of Ice-core, MArine and TErrestrial records) is a core program of the INQUA (International Qua- ternary Union) Palaeoclimate Commission. Dating the Last Glacial-Interglacial Transition 61

Walker et al. (1999a) emphasize that this scheme is not intended as a basis for tuning the records to the GRIP stratotype: “tuning” does not test whether paleoenvironmental records are synchronous, but rather assumes it. Instead, it is suggested that site and/or regional investigations should comprise three stages: 1) the identification of local (climatic) events, based on independent (proxy) evidence, 2) comparison of the local records with the GRIP stratotype, using “marker” events in both, where feasible, and 3) the use of independent dating evidence to establish the degree of synchroneity (or otherwise) between the local and GRIP events. The third step is by far the most difficult to execute, because there is no direct means of establishing the relationship between ice-core and other chronologies. Attempts have been made to effect time-stratigraphic correlations between terrestrial and/or marine LGIT and ice-core records, using calibrated 14C dates (e.g. Lowe et al. 1995; Sirocko et al. 1996; Yu and Eicher 1998; Asioli et al. 1999). The results have been generally encouraging, in that they show a degree of compatibility between the timing and duration of the principal LGIT climatic events. However, the often large errors associated with the 14C dates (generally ±300 yr or even greater, at 2σ, excluding the additional uncertainties introduced by the new INTCAL98 calibration data-set) impart a generally low level of precision to the correlations. Nevertheless the results suggest that 14C dating could provide a basis for time-stratigraphic correlation, at a relatively high level of precision, if materials of sufficient stratigraphical integrity for 14C dating could be found in those LGIT sequences capable of being analyzed at high temporal resolution and if the dendro-based calibration curve, or some alternative offering the same degree of precision and accuracy, extended through the LGIT. An alternative means of establishing the precise ages and durations of events on land or in the oceans during the LGIT is by the use of varved sequences. A number of annually laminated limnic sequences spanning all or a substantial part of the LGIT have been described in recent years, including those of Lake Go ci¹¿ in Poland (Goslar et al. 1992, 1993), several former and extant lakes in Germany (e.g. Hajdas et al. 1993, 1995; Litt et al. forthcoming), the Cariaco Basin in the western Atlantic (Hughen et al. 1998a, 1998b) and Lake Suigetsu in Japan (Kitigawa and Van der Plicht 1998a, 1998b). The problem with these varved sequences, however, is that some may have been interrupted by hiatuses of unknown duration, while most do not extend to the present-day, which makes precise connections to the calendar timescale problematic. Errors associated with the age estimates of LGIT events derived from varve counting are therefore difficult to quantify. In any case, even if it could be established that the varve chronologies are reliable indicators of time, they would provide a chronology for local events only, and could not be used for dating of, and correlation between, more distant sites which do not contain varved sediments. One approach that offers considerable potential as a basis for correlation between LGIT marine, terrestrial and ice-core records, is tephrochronology. In northwestern and central Europe, visible and microscopic layers of several distinctive tephras have been found in LGIT sequences. These include the widely disseminated Vedde Ash, dated by 14C to the “plateau” at 10.4–10.3 14C ka BP (Björck et al. 1992; Birks et al. 1996; Wastegård et al. 1998; Wastegård et al. 2000); the Laacher See Tephra, which is found throughout north-central Europe and dated to around 11.2 ka 14C BP (e.g. Van den Bogaard and Schminke 1985; Hajdas et al. 1995b); the well-dispersed Icelandic Saksunarvatn Ash, dated to 10,210 ± 30 cal BP (corrected dendrochronology—see Gulliksen et al. 1998); and the Bor- robol Tephra, so far found only at sites in Scotland and Northern Ireland, and dated to around 12.26 14C ka BP (Turney et al. 1997; Lowe et al. 1999). Several tephra layers, including the Vedde Ash (Bard et al. 1994) and collectively termed North Atlantic Ash Zone I, occur within LGIT sediment sequences on the floor of the North Atlantic (e.g. Kvamme et al. 1989). At least two of these, the Vedde Ash and Suksunarvatn Ash, have also been found in the GRIP ice-core, the former dated to 62 J J Lowe, M J C Walker

10,240 ± 30 BP and the latter to around 12.0 ka GRIP ice-core years (corrected GRIP chronology from Grönvold et al. 1995; S Johnsen, personal communication). Since each of these tephra layers forms a time-parallel horizon, they not only constitute markers for linking marine, terrestrial and ice-core sequences, but they also have the potential to underpin correlations based on 14C dating or varve chronology. However, the relatively limited number of tephras that have so far been detected, coupled with their restricted geographical distribution, means that this potential has yet to be fully realized. Nevertheless rapid progress is being made in this field, and it is anticipated that tephrochronology will become an increasingly powerful tool in dating and correlating LGIT events (Lowe et al. 1999). That said, however, there still remains a need for a more universal dating method, i.e. one that is applicable to all sites in both the marine and terrestrial realms, and the only method that, at present, has such widespread application is 14C dating.

PROTOCOLS FOR IMPROVING THE PRECISION OF 14C DATES OBTAINED FROM LGIT SEQUENCES It might reasonably be anticipated that the dendro-calibration curve will eventually be extended to the beginning of the LGIT, or that some other, equally detailed and reliable means will be found for the precise calibration of 14C dates beyond the present limit of the dendrochronologically derived curve (Van Andel 1998). Once this has been achieved, it will then be possible to establish, with more confidence, the calendar ages of events represented in marine and terrestrial LGIT sequences, by “wiggle-matching” to the major inflexions. Indeed it appears that such “inflexions”, which reflect sudden changes in atmospheric 14C enrichment (∆14C), were even more pronounced during the LGIT than during the Holocene, because of abrupt changes in ocean ventilation during the LGIT. These would have affected the rate of exchange of 14C between the atmosphere and oceans and hence modulated the effects of the geomagnetic field strength, which seems to be the principal driver of the ∆14C variations (Voelker et al. 1998). Until a more reliable basis for calibration is achieved, however, the question that arises is how best to ensure that the 14C data-sets are of adequate quality for use in calibration procedures. It is axiom- atic that if high-precision dating is to be achieved, those chronologies should only be based on data that meet the highest quality assurance criteria. Yet published reports of 14C chronologies obtained from LGIT sequences rarely provide more than skeletal information about sampling strategies, about the nature of the materials supplied for dating, or about the laboratory protocols and pretreat- ments employed prior to 14C measurement. In too many cases where dates obtained from bulk sediment samples are reported, there appears to have been no, or very limited, advanced testing of the suitability of the materials for 14C dating (measures of the organic C content of the sediments; measures of percent carbonate; “humic” versus “humin” enrichment values, etc.). Often, too, there is little or no information provided on the laboratory procedures employed during the extraction of the material from cores or monoliths, or the length of time that has elapsed between the collection of the samples and delivery to the dating laboratory, of the conditions under which samples were stored, etc. In discussion of the dates, too, it is seldom acknowledged that the quoted error ranges represent only the compounded uncertainties associated with isotopic analyses (and note that these are quoted to 1σ only), and by no means reflects the realistic extent of chronological confidence. Objective interpretation should include some quotient to represent the accumulated effects of geological, sample contamination, and pretreatment uncertainties, although these are rarely quantifiable. True confidence ranges will, therefore, in most instances be significantly greater than the analytical error as expressed in 14C years. Dating the Last Glacial-Interglacial Transition 63

Table 1 Issues to be considered in the design of laboratory protocols and reporting of 14C age determinations that may be used in high-precision dating and correlation 1. Sample integrity (e.g. for bulk organic sediment samples) • provider’s sample number; • single measurement, or part of a series of measurements? if ‘yes’, specify provider and radiocarbon lab. numbers for all other samples in series; • information of stratigraphic (temporal) resolution of sequence from which samples have been obtained; • nature of material dated; • organic C content (LOI); • carbonate content; • nature of any clastic residue; • wet weight of sample submitted to laboratory; • total organic C (dry wt.) used in radiocarbon measurement; • tests for radiocarbon activity heterogeneity of bulk material (e.g. ‘humic’ versus ‘humin’; other fractions tested); 2. Laboratory handling (provider) • date of collection of samples; • date of submission of samples to radiocarbon laboratory; • storage conditions between date of collection and date of submission; • treatment of samples over storage period; • nature of solvents, dispersants (etc.) used; • conditions under which samples extracted and packaged for transfer to radiocarbon laboratory (e.g. whether under controlled air conditions); 3. Radiocarbon laboratory procedures • date samples received; • date samples counted; • storage conditions between date of receipt and date of count start; • pretreatment procedures adopted; • formal laboratory sample number; • count statistics; • corrections applied (e.g. fractionation; reservoir effects); • comments on any unusual chemical effects during pretreatment; 4. Calibration procedures applied • calibration data-set employed (specify version – e.g. CALIB4.0); • specify any smoothing or rounding procedures employed; • single calibration, or part of a series of calibrations? if ‘yes’, specify provider and radiocarbon lab. numbers for all other samples in series.

A distinction needs to be made between dates that give only a general indication of age, and those intended to form a basis for precise geochronologies. Many 14C dates are obtained simply as “rangefinder” age estimates, in order to be able to allocate a stratigraphical unit or horizon to a specific time interval. Such dates were never intended for high-precision reconstruction. Nevertheless, we would contend that, even in such cases, some minimal information ought to be provided about the nature of the materials employed for dating, about the handling of the materials prior to, and during, 14C measurement, and about the counting procedures. Otherwise it is very difficult to assess the reliability of the resulting chronologies, and to recognize where reasonably narrow counting errors are excessively misleading. Where, on the other hand, the objective is to date events or horizons precisely, then there is a greater onus on the operator to provide contextual information about the mate- 64 J J Lowe, M J C Walker rials dated, their stratigraphic integrity, and the laboratory procedures adopted. This becomes crucial in the assessment of 14C dates for inclusion in international databases, the purpose of which is to provide a geochronological underpinning for large-scale paleoenvironmental reconstructions. There is a need, therefore, for the user community to recognize and to adopt more stringent laboratory protocols in order to meet the minimal quality assurance standards that will enable appropriate screening of 14C dates prior to inclusion in international databases. Table 1 (above) lists some of the factors that might need to be considered in the design of such protocols, and includes suggestions as to the sorts of questions that might need to be posed. Some of this information is included in the laboratory reports provided to the user by the 14C laboratory, and subsequently published in Radiocar- bon. However, other equally important information does not appear in these reports, and there is no consistent codification of the contextual data. These and related matters have recently been considered by the INTIMATE international collaboration group (see e.g. Bjorck et al. 1998; Walker et al. 1999a), whose principal aims are to achieve more precise correlations between marine, terrestrial and ice-core records from the LGIT. A series of international workshops over the next four years is designed to further these aspirations and to offer appropriate advice on dating and correlation to those contributing to the work and aims of the INTIMATE program. We would, however, encourage the wider 14C user-community also to consider the need for improved protocols in both field and laboratory sampling, as well as in the reporting of 14C dates. In addition, we would invite reflection on how best to codify the detailed contextual information that might be required for the screening of 14C dates prior to inclusion in the major international databases that are now being assembled.

ACKNOWLEDGMENTS The views expressed here arise from our experiences of working with 14C data-sets on a number of projects funded by the Natural Environment Research Council (NERC). The most recent are: NERC grant GR9/02470; NERC “TIGGER” Special Topic grant GST/02/72 (Geology in the Terrestrial Ini- tiative in the Global Environmental Research Programme) and NERC Special Topic grant GST/02/ 523 (Palaeoclimate of the last glacial/interglacial cycle). We thank NERC for its support. We are also grateful to our collaborators in the INTIMATE program with whom we have shared many stimulating discussions on this and on related topics. We thank Justin Jacyno (Royal Holloway) for producing Figure 1 and Doug Harkness for his constructive critical review of an earlier draft of this paper.

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RADIOCARBON – A UNIQUE TRACER OF GLOBAL CARBON CYCLE DYNAMICS

Ingeborg Levin • Vago Hesshaimer Institut für Umweltphysik, University of Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany. Email: [email protected].

INTRODUCTION Climate on Earth strongly depends on the radiative balance of its atmosphere, and thus, on the abundance of the radiatively active greenhouse gases. Largely due to human activities since the Industrial Revolution, the atmospheric burden of many greenhouse gases has increased dramatically. Direct measurements during the last decades and analysis of ancient air trapped in ice from polar regions allow the quantification of the change in these trace gas concentrations in the atmosphere. From a presumably “undisturbed” preindustrial situation several hundred years ago until today, the CO2 mixing ratio increased by almost 30% (Figure 1a) (Neftel et al. 1985; Conway et al. 1994; Etheridge et al. 1996). In the last decades this increase has been nearly exponential, leading to a global mean CO2 mixing ratio of almost 370 ppm at the turn of the millennium (Keeling and Whorf 1999).

The atmospheric abundance of CO2, the main greenhouse gas containing carbon, is strongly controlled by exchange with the organic and inorganic carbon reservoirs. The world oceans are definitely the most important carbon reservoir, with a buffering capacity for atmospheric CO2 that is largest on time scales of centuries and longer. In contrast, the buffering capacity of the terrestrial biosphere is largest on shorter time scales from decades to centuries. Although equally important today, the role of the terrestrial biosphere as a sink of anthropogenic CO2 emissions is still poorly understood. Any prediction of future climate strongly relies on an accurate knowledge of the greenhouse gas concentrations in the present-day atmosphere, and of their development in the future. This implies the need to quantitatively understand their natural geophysical and biochemical cycles including the important perturbations by human impact. In attempting to disentangle the complexity of these cycles, radiocarbon observations have played a crucial role as an experimental tool enlightening the spatial and temporal variability of carbon sources and sinks. Studies of the “undisturbed” natural carbon cycle profit from the radioactive decay of 14C in using it as a dating tracer, e.g. to determine the turnover time of soil organic matter (SOM) or to study internal mixing rates of the global oceans. Moreover, the anthropogenic disturbance of 14C through atmospheric bomb tests has served as an invaluable tracer for gaining insight into the global carbon cycle on the decadal time scale.

Global Atmospheric Carbon Dioxide Cycle The mean residence time of carbon dioxide in the atmosphere is in the order of only 5 yr. Accord- 9 ingly, 20% of the atmospheric CO2 inventory of today about 750 GtC (1 GtC = 10 metric tons of Car- bon = 1015 g C) are annually exchanged with the biosphere (ca. 60 GtC per year) and the ocean surface waters (ca. 90 GtC per year) (Schimel et al. 1995). Compared to this large gross exchange of carbon between reservoirs, the total yearly net perturbation fluxes are smaller by more than one order of magnitude: In the 1980s, the input of CO2 into the atmosphere from burning of fossil fuels and cement production summed up to about 6 GtC yr−1, and, in addition, about 1–2 GtC yr−1 was released in the course of deforestation and land use change. Although the effect of these emissions on the atmospheric CO2 concentration is clearly observed and well documented, it adds up to a mean increase of only 3 GtC yr−1 in the atmospheric burden during the 1980s (Figure 1a). The remaining fraction, corresponding to about 50% of the fossil fuel emissions, is buffered away about equally by

69 70 I Levin, V Hesshaimer

the world oceans and the biosphere. This partitioning of anthropogenic CO2 between buffer reservoirs is determined by the dynamics of their internal mixing as well as by the strength of their gross carbon exchange with the atmosphere and leads to a residence time of excess man-made CO2 in the atmosphere as large as several hundred years. Despite intensive research in the last decades, the uncertainty of the gross exchange rates between carbon reservoirs is still in the order of ±20%, and it remains difficult to univocally quantify the repartition of net uptake between the biosphere and the oceans, and thus, to predict the fate of anthropogenic CO2 emissions. The situation is further complicated by the fact that the share of excess CO2 taken up by the biosphere and oceans may clearly change in the near future under possibly changing climatic conditions. This concerns in particular the terrestrial biospheric reservoir, which has only a small carbon inventory (about 2200 GtC) compared with the oceans (39,000 GtC). Moreover, this reservoir consists of living plants and dead SOM, which are highly vulnerable and already today strongly perturbed by human activity. The terrestrial biosphere shows a large diversity, ranging from high-latitude tundra and boreal forests, to cultivated land and grassland, to equatorial evergreen forests. In the oceans, the repartition of carbon is less heterogeneous but a large temporal and spatial variability of dissolved inorganic carbon (DIC) is still observed. Therefore, reliable uptake rates of excess CO2 resulting from net changes of the carbon inventory in the oceans and in the biosphere are difficult to obtain by direct long-term observations in these reservoirs. In contrast, the atmosphere, being well-mixed in comparison to the biosphere and the oceans, is presently the best-known of all carbon reservoirs. Sub- stantial information about the carbon cycle has been derived from the interpretation of the observed spatial and temporal variability of atmospheric CO2 in combination with model simulations of the biosphere and the oceans. Consequently, in the past, net anthropogenic carbon fluxes into the biosphere and oceans have been mainly estimated from the gross exchanges between the atmosphere and these reservoirs, also taking into account their internal dynamics. These models have been constrained by suitable (transient) tracers such as bomb 14C, tritium, and chlorofluorocarbons in the case of the oceans, and by natural and bomb 14C in the case of the biosphere. The most prominent examples of the application of 14C will be discussed in the following sections.

Global 14C Suess Effect Derived from Tree-Ring Analyses

It has been erroneously argued that the observed atmospheric CO2 increase since the middle of the 19th century may be due to an ongoing natural perturbation of gross fluxes between the atmosphere, biosphere, and oceans. That the increase is in fact a predominantly anthropogenic disturbance, caused by accelerated release of CO2 from burning of fossil fuels, has been elegantly demonstrated through 14C analyses of tree rings from the last two centuries (Stuiver and Quay 1981; Suess 1955; Tans et al. 1979). Figure 1c shows high-precision 14C data mostly from single tree rings of two Dou- glas-firs grown from 1815 to 1975 at the Olympic Peninsula (48°N, 124°W) about 15 km from the Pacific coast of the United States (Stuiver and Quay 1981). A strong decrease of about ∆14C = −20‰ from 1890 to 1950 is observed in these tree rings. Only a small fraction (∆14C = −3‰) of this decrease is estimated by Stuiver and Quay to be due to natural processes (namely the changing natural 14C production rate through solar and geomagnetic variations), the remaining decrease being attributed to 14 14 14 increasing dilution of C in atmospheric CO2 by input of C-free fossil fuel CO2 (the so-called C Suess effect). Taking into account the fossil fuel emission rates for the time span in question (Figure 1b) the Suess effect could be quantitatively reproduced by these authors (and independently by Siegenthaler [1983]) using a carbon reservoir box diffusion model. The overwhelming effect of bomb 14C on the atmospheric 14C level from 1950 onwards, which cancelled out the 14C Suess effect from 1955 onwards (shaded area in Figure 1c), is discussed in the next section. Radiocarbon as a Unique Tracer 71

Figure 1 (a) Atmospheric CO2 increase derived from direct observations at Mauna Loa station (Keeling and Whorf 1999) and from analysis of air inclusions in Antarctic ice cores (Etheridge et al. 1996). (b) Anthropogenic CO2 emissions from fossil fuel burning, cement manufacturing and gas flaring. (c) Temporal change of ∆14C in tree rings grown at the Pacific 14 coast: The ∆ C decrease is closely related to the increasing anthropogenic CO2 emissions and ∆14 the resulting increase in atmospheric CO2 mixing ratio. (Note: after 1955 the decreasing C trend ends due to the overwhelming effect of bomb 14C input into the atmosphere.)

The ∆14C depletion in the first half of the 20th century has been shown to be larger in more polluted areas, such as Europe, than at the west coast of the United States (De Jong and Mook 1982). In particular, highly populated areas show regional Suess effects up to δ∆14C = −100‰, especially during winter (Levin et al. 1989). A discussion of the use of 14C for regional studies in the context of veri- fying emission reductions (as agreed upon in the Kyoto Protocol), will be given below (Regional Suess Effect section). 72 I Levin, V Hesshaimer

14 Direct Long-Term Observations of Bomb CO2 in the Global Carbon Reservoirs During the atmospheric nuclear weapons tests in the 1950s and early 1960s, large amounts of 14C, in total 630 × 1026 atoms of 14C or more (Hesshaimer et al. 1994), were produced in the atmosphere. 14 14 12 This artificial C input caused a global increase in the C/ C ratio of atmospheric CO2, leading to a substantial disequilibrium of 14C between atmosphere, biosphere, and surface ocean water. In the decade following the testing, many laboratories from around the world conducted large observational 14 programs to document stratospheric and tropospheric CO2 changes as well as the penetration of bomb 14C in all coupled reservoirs of the atmospheric carbon cycle. These measurements enable us to use 14C as a unique tracer for carbon transfer processes between the reservoirs in question.

14 More than 30 years of direct tropospheric CO2 observations from both hemispheres are now available since the nuclear test ban treaty in 1962 (Figure 2a) (Tans 1981; Nydal and Lövseth 1983; Levin et al. 1985, 1992; Manning et al. 1990; Meijer et al. 1995). Also, the stratosphere has been extensively sampled during and after the atmospheric bomb tests. The latter data were originally gathered by Telegadas (1971) who calculated bomb 14C inventories for eight major sub-compartments of the stratosphere. A reassessment of the same data set was performed recently by Hesshaimer and Levin (2000), providing revised stratospheric inventories suitable for global carbon cycle and transport modeling (Figure 2b). The bomb 14C decline observed in the troposphere mainly reflects the pene- 14 tration of the CO2 perturbation into the oceans and the biosphere, which is essentially driven by the internal circulation dynamics within each of these two reservoirs. But also, ongoing fossil fuel CO2 emissions and 14C sources such as nuclear facilities (Otlet et al. 1992, and references therein), and possibly nuclear underground detonations, contribute to the observed trend.

14 The oceans have been extensively sampled for CO2 analysis in DIC during several international surveys: the Geochemical Ocean Sections Study (GEOSECS, 1972–1978), the Transient Tracers in the Ocean (TTO, 1980–1982) study, and the South Atlantic Ventilation Experiment (SAVE) are prominent examples in this context. In addition, the time development of 14C in surface ocean water since preindustrial times, and including the bomb perturbation (Druffel and Suess 1983; Druffel 1995) was derived from corals that accrete annual density bands with 14C activities equal to those of DIC in the surface water of the oceans (see Figure 2a). These data have been evaluated by Broecker et al. (1985, 1995) to determine the distribution of the natural 14C component in DIC as well as the total bomb 14C inventories of the world oceans. Their best figure of the global oceanic bomb 14C inventory, normalized to 1 January 1974, amounts to 295 ± 30 × 1026 atoms of 14C (Broecker et al. 1995) (see Figure 2b).

Air–Sea Gas Exchange Rate and Internal Mixing of the Oceans The combined analysis of the development of bomb 14C in the global atmosphere as well as in the surface and deep oceans has given us insight into the strength of CO2 exchange between the atmosphere and the oceans, as well as into the mixing dynamics within the oceanic reservoir itself. Accu- rate knowledge of these processes is crucial to determine the oceanic uptake of anthropogenic CO2. The transport of excess CO2 from the atmosphere into the interior of the oceans is limited by two kinds of transport resistance: air–sea gas exchange which is controlled by a thin water film at the water surface, and oceanic “mixing” circulation responsible for the exchange between the surface and deeper layers of the oceans. In many areas of the oceans, i.e. in mid-to-low latitudes, the gas exchange at the air–sea interface is fast enough, respectively, surface water renewal through mixing e.g. with the deep ocean is slow enough to ensure surface water to be almost in equilibrium with atmospheric CO2 (the global mean equilibration time of ocean surface water is about 1 yr). How- ever, in regions of the ocean with large deepwater formation rates, and respective fast renewal of the Radiocarbon as a Unique Tracer 73

∆14 Figure 2 (a) Long-term observations of C in atmospheric CO2 in the northern and in the southern hemisphere (Manning et al. 1990; Levin et al. 1992 and unpublished Heidelberg data). Shortly 14 after the atmospheric test ban treaty in 1962, the CO2 level in the northern hemisphere was twice 14 ∆14 as high as the natural CO2 level (defined as 0 in the C scale [Stuiver and Polach 1977]). Also included is the ∆14C level of DIC in surface ocean water between 17°N and 25°N derived from annual banded coral rings (Druffel and Suess 1983; Druffel 1995) together with model calculations for the surface ocean by Hesshaimer (1997). (b) Temporal trend of the observed bomb 14C inventories of the stratosphere up to 30 km (Hesshaimer and Levin 2000), the troposphere (derived from the observations in [a]) the ocean (box-diffusion model estimates by Hesshaimer et al. [1994], tuned to fit the total inventory of Broecker et al. [1995]) and the biosphere (3-box compartment model estimates by Hesshaimer et al. [1994]). surface water (i.e. in polar regions), the rate of gas exchange becomes important for net transport of excess CO2 into the deep ocean (Siegenthaler 1983). Both natural and bomb 14C have been successfully used in the past to derive the mean atmosphere/ ocean gas exchange rate for major ocean basins (Stuiver 1980; Stuiver et al. 1981). The net 14C 14 influx from the atmosphere to the surface ocean at a given time is proportional to the ∆ C gradient 74 I Levin, V Hesshaimer

at the interface multiplied by the gross CO2 exchange flux. In the undisturbed natural case, the global mean difference of ∆14C between the atmosphere and the surface ocean was about 40‰. At equilibrium, the corresponding net 14C influx was just enough to compensate the 14C decay in the oceans, immediately yielding the CO2 gas exchange flux for the preindustrial situation (ca. 280 ppm). During the period after 1955, disturbed by nuclear tests, the CO2 gas exchange rate could be determined in an independent manner from the, in this case, time dependent uptake of bomb 14C in the oceans until the time of GEOSECS (see Stuiver 1980). In this way, Stuiver et al. (1981) derived mean area-weighted CO2 exchange fluxes for the Pacific and for the Atlantic oceans between 50°N and 50°S for 1955–1974. The bomb 14C-derived preindustrial gas exchange rate turned out to be in very good agreement with the mean steady state value. However, the 14C-derived gas exchange rates are larger by more than 30% if compared to figures derived from direct measurements in wind tun- nels, over lakes and open ocean (Liss and Merlivat 1986; Boutin and Etcheto 1997). Although this discrepancy has not been fully resolved yet (Broecker et al. 1986; Wannikhof 1992), the 14C-derived exchange rate between the atmosphere and the ocean, leading to a gross exchange flux of about 90 GtC yr−1 for the period 1980–1990 is adopted for all carbon-cycle models. The dynamic model parameters for the internal mixing of the ocean have also been frequently determined by the distribution of natural 14C. Unfortunately, this procedure gives quite different answers for these fluxes than calibration with the bomb-14C distribution (Oeschger et al. 1975; Broecker et al. 1980; Siegenthaler 1983). The explanation for this discrepancy is that the distribution of natural 14C is more sensitive to mixing processes on time scales of hundreds or thousands of years, whereas the characteristic time scale for the bomb 14C uptake perturbation is only several decades. Siegent- haler and Joos (1992) solved this problem with their High-Latitude Exchange/Interior Diffusion- Advection (HILDA) model. They introduced an eddy diffusivity in the interior of the ocean, which is decreasing with depth. (In the original Oeschger-type box diffusion models, the eddy diffusivity is assumed constant with depth.) With this modification, natural plus bomb 14C-calibration yields a reliable description for the whole range of mixing processes in the ocean on all relevant time scales from 10 to 1000 yr. In particular, the calibration now provides a realistic description of the time- dependent uptake rates for anthropogenic excess CO2 for the time scale in question, namely several hundred years.

Budgeting Bomb 14C in the Global Carbon Cycle Natural and bomb 14C have also been used in several studies to determine carbon turnover times within the terrestrial biosphere, in particular for SOM (e.g. Goudriaan 1992; Trumbore et al. 1993; Perruchoud et al. 1999). However, the heterogeneity of the terrestrial biosphere hampers the determination of globally valid bomb 14C inventories for this reservoir based on observational data. Evi- 14 dence on the exchange fluxes of bomb CO2 with the biosphere is, however, also inherent to the 14 atmospheric CO2 record. It should, therefore, in principle be possible to gain insight into the cycling time of carbon within the biospheric reservoir by budgeting bomb 14C in the global carbon system. This approach was used by Hesshaimer et al. (1994): The total decay-corrected bomb 14C input into the carbon system at a given time must equal the sum of the corresponding bomb 14C inventories of the stratosphere, the troposphere, the oceans, and the terrestrial biosphere. Besides the biospheric bomb 14C inventory, the second unknown in this exercise is, however, the 14C input from atmospheric bomb tests, i.e. the 14C yield per Mt TNT. Theoretically, this 14C yield can be derived from nuclear physics. However, those calculations have uncertainties in the order of nearly a factor of 2 (Bonka 1980; UNSCEAR 1982). The total bomb 14C input into the atmosphere was, therefore, derived by Hesshaimer et al. (1994) on the basis of published bomb detonation strengths (Rath 1988) through adjusting the 14C yield per Mt TNT to the tropospheric and stratospheric observations Radiocarbon as a Unique Tracer 75

14 during the time period of the major CO2 rises (i.e. 1950–1963). As long as we trust the atmospheric observations and the bomb detonation strengths the uncertainty of this adjustment is small (in the order of about ±10%) since the total bomb 14C uptake by the ocean and by the biosphere was not important before 1963. A simple standard compartment model of the global carbon cycle (similar to the one by Oeschger et al. 1975) was then applied to infer the straight forward bomb 14C budgeting. In doing so, Hesshaimer et al. (1994) found a serious mismatch in the total bomb 14C inventory with an apparent 14C source missing. Possible solutions for this mismatch have been argued to be due to: 1) an over-estimate of the oceanic bomb 14C inventories, 2) residing bomb 14C hot spots in the very high stratosphere, or 3) much less bomb 14C uptake by the terrestrial biosphere than estimated from standard parameteriza- tions of this reservoir (e.g. Goudriaan 1992). Other modeling studies, trying to globally match bomb 14 CO2 levels observed in the carbon cycle reservoirs (Enting et al. 1993; Broecker and Peng 1994; Lassey et al. 1996: Jain et al. 1997) solved inconsistencies in their budgets by generally mistrusting the stratospheric inventories of Telegadas (1971). These inventories have turned out to be largely confirmed by the revised calculations of Hesshaimer and Levin (2000) so that further investigations are necessary to solve the problem of budgeting bomb 14C in the global carbon cycle. Solving this inconsistency, and in particular, independently confirming the penetration rate of bomb 14C into the oceans, is crucial for the prediction of future atmospheric CO2 concentrations because the oceanic 14 uptake of excess CO2 is generally normalized with bomb C (see the Air–Sea Gas Exchange section above). As mentioned earlier, it is important to know explicitly which fraction of the anthropogenic emissions enters the biosphere and which fraction enters the oceans, because carbon storage in these two reservoirs behaves very differently. If the anthropogenic CO2 is eventually stored in the deep oceans it is unlikely that it will re-enter the atmosphere within the next centuries. In contrast, CO2 stored in the biosphere may be released back to the atmosphere within a few decades. Moreover, the capability of the terrestrial biosphere for uptake of excess-CO2 is very sensitive to changes through human activities, in supply of nutrients and water, but also due to climatic feedback.

14 Global Distribution of Atmospheric CO2 in the 1990s and Beyond The most prominent applications of the bomb 14C perturbation as a transient tracer for carbon cycle studies took place in the 1970s and 1980s when the signal was largest in all reservoirs. After 1990, 14 many of the extensive observational programs phased out as CO2 approached a new quasi-natural ∆14 equilibrium state. From 1982 onwards the globally decreasing CO2 trend in fact almost exactly follows an exponential curve with a time constant of 18.70 ± 0.15 yr, thus changing from a decrease 14 −1 −1 of about ∆ CO2 = −13‰ yr in 1982 to about −4‰ yr in 1998 (see Figure 2a). With the last atmospheric bomb test being conducted in 1980, and although much weaker underground tests are still performed, bomb 14C homogenized in the troposphere and additional input from the stratosphere 14 became negligible. In the 1990s and today CO2 in the troposphere, therefore, again reflects the distribution of natural and anthropogenic CO2 sources and sinks. This is nicely illustrated by our modern high-precision measurements at seven globally distributed background stations located between 82°N (Alert, Nunavut, Canada) and 71°S (Neumayer, Antarctica) (Figure 3b). Although the north- south gradients are in the order of only a few per mil, a distinct structure is observed in the yearly 14 mean CO2 profile.

14 Figure 3b shows a relative ∆ CO2 minimum in mid-northern latitudes. It is caused by the maximum effect of fossil fuel CO2 emissions in this latitude belt. These anthropogenic emissions are also responsible for the north-south gradient of the CO2 mixing ratio shown in Figure 3a, with about 1% 14 higher concentrations in the northern than in the southern hemisphere. A second relative ∆ CO2 minimum is observed in mid-to-high latitudes of the southern hemisphere and is associated with gas 76 I Levin, V Hesshaimer

Figure 3 Mean meridional profiles 1993–1994 of (a) CO2 concentration (data from the NOAA/ ∆14 CMDL global network [Tans et al. 1996]) and (b) CO2 in the atmosphere (Heidelberg unpublished ∆ δ∆14 data). Plotted are in (a) and (b) the deviations CO2 and C from the global mean values. (c) 14 ∆ CO2 of DIC in surface ocean water derived from cruises of the TTO experiment (Broecker et al. 1995) together with unpublished Heidelberg data collected in 1986 in the South Atlantic ocean during the Polarstern cruise ANT III. The solid line represents a spline through the 1986/1988 data. exchange with the circum Antarctic ocean surface: upwelling of intermediate water around Antarctica, which is depleted in 14C (Figure 3c), causes a strong disequilibrium between 14 14 atmospheric CO2 and C in DIC of the surface ocean water. The respective CO2 flux from the ocean to the atmosphere is, therefore, depleted in 14C. Driven by the large gross exchange rate of 14 CO2 at these latitudes due to high wind speeds, a strong C draw down is observed in the atmosphere at Macquarie Island and Neumayer. The two ∆14C depleting effects in mid to high 14 latitudes of both hemispheres cause a relative ∆ CO2 maximum to appear in tropical and Radiocarbon as a Unique Tracer 77 subtropical regions (Figure 3b). This maximum is probably further enhanced by a net flux of bomb 14C from the terrestrial biosphere to the atmosphere; two-thirds of the yearly terrestrial biospheric CO2 exchange flux is located between 30°N and 30°S, the mean carbon turnover time of these ecosystems (i.e. tropical rain forests) being in the order of 30 yr. Bomb 14C that was taken up in the early 1960s is now re-emitted to the atmosphere in the 1990s (see Figure 2b). This 14C enriched 14 biospheric CO2 probably contributes to the ∆ CO2 maximum observed at our tropical and 14 subtropical sites. Although the ∆ C signals in recent global background atmospheric CO2 are rather small, they provide independent information on global CO2 sources and sinks. This information has not yet been retrieved from the data sets by carbon-cycle modeling exercises.

Figure 4 (a) Monthly mean ∆14C observations in the upper Rhine valley at the polluted site Heidelberg together with the maritime background level, derived from observations at the baseline station Izaña, Tenerife (28°N) (Levin et al. 1989 and unpublished Heidelberg data). The maritime background 14C level before 1988 is calculated from observations at Vermunt, Austria (47°N) (Levin et al. 1985), which were corrected by ∆14C = +2‰, therewith accounting for a general European fossil fuel offset. (b) Mean fossil fuel contribution at Heidelberg calculated with a two-component mixing model from the ∆14C difference between Heidelberg and Izaña, the latter representing the maritime background level.

Regional 14C Suess Effect

As has been discussed above, the most important man-made perturbation of the atmospheric CO2 cycle is the burning of fossil fuels, with about 95% of the CO2 being released in the northern hemi- ∆14 sphere. The fossil emissions today contribute about 50% to the decreasing CO2 trend observed in the global atmosphere (Figure 2a). In polluted areas, e.g. over the European continent, an additional ∆14 regional surplus of CO2 from the burning of fossil fuels is clearly detectable as C depletion relative to maritime levels. Levin et al. (1995) used ∆14C observations to estimate the fossil fuel contribution to the continental CO2 offset at Schauinsland station (Black Forest, Germany) to be around 78 I Levin, V Hesshaimer

1–3 ppm, depending on season. In more polluted areas, such as the upper Rhine valley in Heidel- berg, the monthly mean fossil fuel contribution can be as large as several 10 ppm (Levin et al. 1989).

14 Figure 4a shows the updated ∆ CO2 record for the Heidelberg site. Particularly during the winter months, mean ∆14C depletions in the order of 50‰ are frequently observed. These translate into mean fossil fuel contributions of 20 ppm during this time of the year (Figure 4b). In fact, during summer, the fossil fuel contributions are generally expected to be much lower due to enhanced vertical mixing and dilution of ground-level pollutants but also due to reduced emissions (no domestic heating). The interannual changes of the yearly mean regional fossil fuel contributions are small, however, and a general trend, i.e. towards decreasing offsets, cannot be detected in the last 20 years. In fact, from statistical emissions data of southern Germany no significant trend should be expected from 1987 to 1996. The mean values for the five-year periods from 1987 to 1991 (8.98 ± 0.31 ppm) and from 1992 to 1996 (8.90 ± 0.58 ppm) compare extremely well within ±1%. This opens the possibility of using regional ∆14C observations to validate fossil fuel emissions reductions, e.g. in the frame of the Kyoto Protocol; here, Germany and the European Union as a whole are obliged to reduce their emissions of CO2 and other greenhouse gases by 8% by 2008–2012, a reduction that will be clearly visible and quantifiable using our 14C-based regional observations in the time frame of a five-year period.

CONCLUSION The radioactive lifetime of 14C is perfectly suited for dating of carbon pools interacting with the atmospheric CO2 reservoir on the time scale of several hundred to several thousand years. The above examples clearly show that our quantitative knowledge of the present gross and net fluxes of CO2 between the dominant carbon reservoirs is significantly based on respective information provided through radiocarbon observations. In particular the use of bomb 14C as a transient tracer in the carbon system provides invaluable insight into processes on the time scale where the largest man made CO2 perturbations took place, namely the last 50–100 yr. The most prominent example is the air–sea gas exchange and the penetration of human CO2 disturbances from the surface into deeper 14 layers of the oceans. Finally, the unique characteristics of fossil fuel derived CO2 being C-free allows the tracking of respective emission changes in the past and also in coming years when reliable tools are needed to validate national emission claims of greenhouse gases in the frame of the Kyoto negotiations.

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AN OVERVIEW OF 14C ANALYSIS IN THE STUDY OF GROUNDWATER

Mebus A Geyh Section 3 – Dating and Isotope Hydrology, Institute of Joint Geoscientific Research, Hannover, Germany. Email: [email protected].

ABSTRACT. This paper provides a summary overview of the current state-of-art in the radiocarbon dating of groundwater. While the use of natural 14C measurements in applied hydrogeology still presents a difficult challenge, meaningful dates can be achieved if they are determined and interpreted in conjunction with the analyses of other isotopic species that occur in the natural environment. Although 14C dating of groundwater can be, and often is, carried out as a matter of routine, any specific case study requires its own scientific design and effort. As is widely recognized, and discussed in considerable detail throughout the scientific literature, there are many hydrogeochemical reactions and/or physical processes that can alter the natural 14C enrichment measured in environmental materials. Fortunately, for fresh groundwater resources such effects are in general well defined and therefore of limited significance. The primary challenge in applied groundwater dating is with the development of the appropriate theoretical background against which 14C dates can be used to calibrate numerical analogues of the groundwater system. The hydraulic properties of each of the widely used finite-element models can be well estimated from numerous piezometric data and extrapolations. In contrast, only a few groundwater ages can be provided for the calibration of those models that are complex functions of aging mixture and sometimes also hydrochemical reactions.

INTRODUCTION The application of conventional radiocarbon dating using the inorganic carbon contained in speleothems was first proposed by Franke (1951). Soon afterwards Deevey et al. (1954) reported a significant reduction in the 14C specific activity of the dissolved inorganic carbon (DIC) compounds in lake water due to the participation of geologically old carbonate where this occurs within the catchment—the so-called “hard water effect” or “reservoir effect” as was later coined by Olsson (1980). The direct dating of groundwater based on the measured 14C activity was introduced by Muennich 14 (1957, 1968). However, it soon became evident that the initial C activity (No) imparted during groundwater recharge is determined in large part by the ongoing hydrogeochemical mass balance reactions towards establishment of the carbonate/CO2 equilibrium state. In general, therefore, the 14 appropriate No value used to determine a C age for the DIC tends to be set at less than 100% modern carbon (pMC) i.e., the theoretical equivalent to time zero on the conventional 14C time scale. By international convention the 14C age (t) in years BP (before AD 1950) of groundwater DIC is calculated from its measured 14C activity (N) and according to the relationship

t = τ/ln2 ln(No/N) , (1) where τ is set at either the Libby half-life of 14C (5570 yr) or the physical half-life of 5730 yr.

14 In addition to any initial uncertainty over the actual C activity (No) set at recharge, the apparent decrease in 14C activity due to radioactive decay during subsequent storage in the aquifer can also be distorted by the superimposed influence of hydrochemical reactions, physical processes, and geohydraulic mixing. Consequently, the great challenge in applied groundwater dating is to resolve and quantify the true age controlled effect of radioactive decay from other possible influences on the measured 14C activity.

HYDROCHEMICAL REACTIONS Objective 14C dating of groundwater requires recognition and quantification of those hydrochemical reactions that will have altered the 14C activity of the DIC in addition to the immutable rate of radioactive decay. Quantification of the isotopic composition from such processes is essential since ulti-

99 100 M A Geyh mately their collective influence will determine the 14C activity of the DIC sample and hence the most appropriate method for its numerical conversion to a meaningful age for the water. There are two stages in the aquifer recharge and storage processes that have to be accounted. 1. First are those reactions that occur during groundwater recharge. These take place in the unsaturated zone that represents an open system for isotopic exchange via the soil zone and the free atmosphere. The general trend is towards a lowering of the 14C activity imparted to the DIC. The extent to which this initial dilution occurs is in turn dependent on the geochemical and pet- rological characteristics of the catchment. 2. Once transferred to the saturated zone of the aquifer, the groundwater can be subject to secondary hydrochemical reactions. Any resultant isotopic alterations that take place during this period of groundwater storage and aging do so within the constraints of a closed geochemical system.

The Initial 14 C Activity of DIC When rainwater enters the topsoil, it encounters an atmosphere that contains very high concentrations of CO2 generated by the biological decomposition of dead organic matter and/or root respira- tion. Dissolution of this CO2 produces hydrocarbonic acid which will in turn react with any soil carbonate present until either calcite saturation or CaCO3/CO2 equilibrium is reached.

− 2++ CO2 + H2O + CaCO3 ↔ 2HCO3 + Ca (2) − In the pH range 6–8, which is common for fresh groundwater, the DIC consists of HCO3 and CO2. The relative concentration of these components is determined and controlled by the partial pressure of CO2 in the topsoil and the ambient temperature. It is necessary to distinguish between open and closed system conditions (Wigley 1977). In open systems, calcite saturation is reached by utilization of the essentially unlimited reservoir of carbon contained as soil CO2 within the unsaturated zone. On entering the closed geochemical system, represented by the saturated zone, the groundwater is separated from this open CO2 reservoir and calcite saturation is approached when reactable dissolved CO2 is in equilibrium with the carbonate ions. Under most natural conditions these hydrochemical reactions actually occur in geochemical states that are transitional between open and closed systems.

Geochemical Modeling

The initial reactants in the CO2 /carbonate exchange have different origins and consequently quite distinct carbon isotope signatures i.e., specific 14C activity and the stable-isotope composition 13 14 (δ C). The biogenic derived CO2 has a C activity close to 100 pMC (percent modern carbon) and 13 13 13 a δ CPDB value of about −23‰. C is enriched (less negative δ C) compared to common terres- 13 trial organic matter since atmospheric CO2 with a δ CPDB value of −8 to −7‰ diffuses into the top- 13 soil. Soil lime is considered to be of marine origin with a δ CPDB value close to 0‰ and is so old as to be essentially free from 14C i.e., an activity of 0 pMC. Therefore, it is the relative contribution from dissolved bicarbonate and carbon dioxide that determines the carbon isotope composition that characterizes the DIC in freshly recharged groundwater. The initial 14C activity ranges from 80 to 100 pMC, and from 54 to 84 pMC for open and closed con- 13 ditions, respectively. The corresponding ranges of stable isotope composition (δ CPDB) are −17 to −16‰ and −13 to −12‰ (Clark and Fritz 1997). 14C Analysis of Groundwater 101

Various theoretical models have evolved to estimate appropriate values of No based on hydrochemical evolution processes in conjunction with the stable carbon isotope composition of DIC (Mook 1976; Clark and Fritz 1997). The approach was pioneered by Ingerson and Pearson (1964) who pro- 13 posed the quantification of No as a function of the δ CPDB values of the DIC, soil CO2 and soil lime. 14 This basic model assumed closed system conditions (100 pMC for soil CO2 and C free carbonate, and perfect stoichiometry for carbon exchange). Gonfiantini (1972) replaced the relationship and introduced the isotope fractionation (ε) between dissolved bicarbonate and CO2. The appropriate value of ε depends upon the temperature and pH; it ranges from +7 to +10‰. In this instance:

13 13 13 13 No = N(bio) (δ CDIC − δ Ccarb)/(δ Cbio + ε − δ Ccarb)(3)

Where Nbio is assumed to be 100 pMC and the indices “bio” and “carb” refer to the soil CO2 and soil carbonate, respectively. Geyh and Wendt (1965) introduced the concept of chemical balance for the numerical estimation of − No. The method is dependent on knowing the concentrations of CO2 and HCO3 in the groundwater, though in practice it is difficult to determine that of CO2 precisely. Tamers et al. (1967) improved this approach by replacing the CO2 concentration term by the difference between total dissolved − inorganic carbon (TDIC) and the HCO3 concentration. Both models assume closed system conditions. More complicated process-oriented models that recognize both the hydrochemical balance and the isotope fractionation have been developed subsequently (Mook 1976; Reardon and Fritz 1978). Fontes and Garnier (1979) even included mixing–matrix exchange, i.e. rock/water interactions. A computer program (NETPATH) has been developed (Plummer et al. 1994) to simulate the pro- gressive changes in carbon isotope geochemistry along the groundwater flow path based on the prevailing hydrochemical conditions.

Limitations of the Theoretical Models

Processes and Model Conditions. It must be recognized that in practice the objective application of any theoretical model is constrained. A first consideration is the fact that most natural groundwater systems tend to exhibit the complex interaction of several hydro-geochemical and/or physical mixing processes. Moreover, calcite saturation and isotopic equilibrium, although often assumed in theory, are not always achieved in nature. Consequently, although the overall effect of multiple processes in determining the initial 14C activity of DIC can be quantified as the product of the individual dilution factors (Clark and Fritz 1997), it is important to recognize that the preferred isotopic and hydrochemical initial field parameters employed in the calculation are seldom known precisely. Objective application of the NETPATH program (Plummer et al. 1994) is for example limited by the prerequisite that the pattern of samples intended for 14C measurement must reflect a hydrologic flow path which in itself cannot be located precisely.

Isotopic Field Parameters. It is presumed in all models that the initial 14C activity of the participat- ing soil carbonate is zero. However, Geyh (1972) showed that the 14C enrichment of soil lime from immediately below the decalcified zone can exhibit values as high as 75 pMC and a mean of 15 pMC obtains in humid regions. During the warm season the evaporation of recently recharged groundwa- 102 M A Geyh ter tends to result in a precipitation of calcite crystals and an accumulation of excess 14C in the unsaturated zone. Freezing in the unsaturated zone can have a similar effect. In coastal areas the predominant contributor to the carbonate reservoir contained in the topsoil is young mollusc shells which are relatively rich in natural 14C. Similarly, in karst areas, Holocene aged tufa may be exposed and/or present as carbonate filling in geological fractures. Likewise, it is possible that weathering of Felspar by the action of biogenic CO2 will add small amounts of bicarbonate with a 14C activity of 100 pMC to the DIC. However, as was highlighted by Ehhalt and Vogel (1963), this is a very slow, and therefore relatively insignificant process. In such instances the reservoir-effect corrected 14C ages of groundwater will be apparently too small. Tamers et al. (1975) have contended that the realistic precision that can be achieved in groundwater dating is constrained to a considerable degree by any thorough error analyses that recognizes the ranges of 14C activity and 13C enrichment that can obtain for the various carbonaceous parameters involved. This opinion is well exemplified via the frequently used “Gonfiantini model” (Equation 2). If we adopt an uncertainty range of ±2‰ for the δ13C values that are assumed for both the biogenic CO2 and the soil carbonate and ±1‰ for DIC, then the modeled dating precision for Holocene groundwater will be ±2700 yr. This in stark contrast to the ±1000 yr (1 σ) error associated with and arising from the conventional 14C age measurement. In practice, however, the results of many case studies show that the scatter of 14C groundwater dates is far less—generally within a ±500-yr enve- lope (Geyh 1992), the reason being that although the scatter of the initial δ13C values encountered in restricted recharge areas is not accurately known, this is nevertheless of significantly less magnitude than the global ranges.

While the basic models tend to assume that soil CO2 is entirely of biogenic origin and hence characterized with a δ13C value of –25‰ there the is an almost inevitable ongoing exchange with atmospheric CO2 which requires a more realistic value of –22‰. Furthermore, in arid and semi-arid regions, C4 type vegetation (sugar cane, corn, sorghum, grasses in the savanna) is liable to dominate 13 13 the soil CO2 and imprint much heavier C enrichment (typically δ CPDB− = −13‰). Usually the vegetation consists of an unknown assemblage of both types of plants and with relative variations in previous years. Where bacterially mediated reactions occur, these are invariably accompanied by large shifts in isotopic fractionation. The prime example here is methanogenesis (Clark and Fritz 1997). Since the extent of the induced isotopic fractionation is generally site specific and unknown in most instances it is not possible to adjust the measured radiometric (14C) enrichment of the DIC to achieve a realistic 14C date.

Empirical Approaches

Many empirical approaches have been applied to estimate No and hence determine directly the calibration factor used to define the assumed age of DIC in groundwaters. Frequently this is simply use of the fixed correction value of 85 pMC as proposed by Vogel and Ehhalt (1963).

As an alternative, Geyh (1972) calculated a range of No values that would be more applicable to describe the initial 14C activity of DIC in spring water from specific geological settings. These estimates (Table 1) are often found to compare well with the more stringently determined No values modeled independently and in relation to other isotopic parameters. Another relatively simple approach to determining the appropriate “reservoir correction” value was proposed by Vogel (1970). The procedure involves measurement of the conventional 14C age of 14C Analysis of Groundwater 103

DIC sampled at recorded intervals along the groundwater flow path in confined aquifers and the subsequent linear extrapolation of these data back towards the catchment region where the actual water age can be assumed at close to zero. The method is exemplified in Figure 1.

)

1 (

C velocity = 0.66 (m yr-1)

o i

t 10

o c

reservoir correction = -1300 (yr) 0 0 5101520 distance from outcrop (km) Figure 1 Increase in 14C ages of DIC in groundwater as a function of distance from the catchment area (after Vogel 1970) as used to estimate the initial 14C activity

Table 1 Initial 14C activity values and corresponding reservoir (age) corrections for different geological settings (Geyh 1972) Initial 14C activity Reservoir age correction Catchment geology (pMC) (yr BP) Crystalline 90–100 −1000 to zero Loess covered 85 −1300 Uncovered karst, dunes 55–65 −5000 to −3500

Another very successful approach, developed by Verhagen et al. (1991), is summarized in Figure 2. It is applicable in systems where the groundwater may have incorporated post-nuclear CO2 i.e., as evidenced by the occurrence of measured 14C activities greater than 100 pMC, and involves the construction of a “3H/14C” or alternatively a “85Kr/14C” diagram. The appropriate initial 14C activity is then assumed to occur where the curve intersects the tritium detection limit. The basic reasoning here is that a groundwater sample that does not contain “bomb” tritium will also be free of anthropogenic 14C. In fractured aquifer systems (open system) the mean residence time (MRT) rather than the water age of groundwater may be estimated by applying the exponential model to the specific activity of tritium and 14C. In the case of 14C there are two unknown parameters, viz., the initial 14C activity and MRT. Since any spring or well-water sample has its specific MRT, by applying this method to the measured 3H and 14C activities of several samples from the same catchment, the initial 14C activity is obtained in addition to the MRT (Geyh 1972). 104 M A Geyh

)

(

e 15

H 3 10

initial 14C activity (pMC) 5

0 60 80 100 120 14C activity (pMC) Figure 2 3H/14C diagram as used to estimate the initial 14C activity in DIC of groundwater from the northern Kalahari (Ver- hagen et al. 1974)

Figure 3 Application of different hydrochemical models to estimate the initial 14C activity of DIC in groundwater from the Ad Rhuma aquifer in Oman based on palaeohydrological information (after Clarke et al. 1996)

In favorable circumstances, paleohydrological, paleoclimatological, and prehistoric information can also be usefully employed in estimating the initial 14C activity or to check its accuracy (Geyh 1992; Clark et al. 1996). An example that incorporates this approach as described by Clark et al. (1996) is summarized in Figure 3 above. In this instance, supplementary correction models were applied to account for the influence of both carbonate dissolution and sulphate reduction (Clarke and Fritz 1997) and to ensure that the 14C ages of the DIC corresponded with the paleohydrological situation. 14C Analysis of Groundwater 105

Rogojin et al. (1998) used the measured 234U excess to calibrate the 14C timescale of groundwater. A good correlation was established between the uranium isotope ratios and 14C ages of DIC in groundwaters sampled from the oxygenated parts of a limestone and a sandstone aquifer in Israel.

Natural 14C Activity in DOC Dissolved organic carbon (DOC) in groundwater consists of organic liquids, hydrocarbons, methane, and humic components. It is produced in soils and peat layers by microbacterial degradation of organic detritus and via the oxidation of lignite or kerogen. The youngest recognized constituent of groundwater DOC is the group of organic compounds classed as fulvic acids (FA). These are the most promising molecules for dating groundwater DOC (Geyer et al. 1993; Aravena et al. 1993). The more abundant humic acids (HA) are less suitable. In general, the concentration of fulvic acids in groundwater is low, often providing as little as 1 mg C/liter. Since the component carbon of fulvic acids can derive from a variety of pedogenic and/or geogenic sources all of which are of potentially variable age, the composite natural 14C activity (N) can be expected to be significantly lower than 100 pMC. Geyer et al. (1993) reported values from 34 to 100 pMC but noted the greatest frequency of results occurred within the range 75–100 pMC. Given this situation, any quantification of an appropriate No value (see Empirical Approaches, above) must be recognized as an empirical estimate. Therefore, the initial expectation that conventional 14C dating of DOC could overcome the hydrogeochemical problems inherent in dating DIC has not been fulfilled. Nevertheless, comparative dating of DOC is often a useful supplement to parallel dating of the component DIC in selected groundwaters.

Secondary Reactions While hydrochemical reactions in the saturated zone can reduce the residual 14C activity of DIC independent of, and in addition to, the ongoing radioactive decay process, DOC is not susceptible to such reactions. However, it must also be recognized that the possible admixture into the bulk DOC of fossil derived fulvic acid will signal erroneously old 14C ages (Geyh 1991).

Oxidation of Organic Matter and Subsequent Carbonate Dissolution The most serious and frequently observed hydrochemical reaction that can disturb the 14C activity of DIC is the formation within the saturated zone of CO2 from fossil organic matter contained in the aquifer system. This reaction is facilitated by the consumption of dissolved oxygen (up to 6 mg/liter) or the reduction of any sulphate and/or nitrate present. Once formed, the CO2 is likely to enable the dissolution of additional fossil carbonate. Many aquifers contain fossil organic material and many groundwaters exhibit high concentrations of sulphate. Where such conditions prevail, the measured stable isotope enrichments (δ13C values) are no longer applicable for either estimating the initial 14C activity (Figure 4) or to correct for secondary non-decay changes to the 14C activity of the DIC. The impediment here stems from the fact that there are two independent sources of carbon with diver- gent δ13C and 14C activity values viz., fossil organic matter with respectively −25‰ and 0 pMC, and soil carbonate with respective isotope signatures of 0‰ and 0 pMC. As is shown in Figure 4, both the apparent age and concentration of DIC increases due to the input of fossil derived CO2 while the coincident δ13C values change in an irregular pattern (Geyh and Kantor 1998).

Clarke and Fritz (1997) proposed a correction scheme for this reaction based on the measured H2S concentration, but the precise determination of this parameter is difficult in groundwater samples. A correction scheme proposed in 1972 by Hans Oeschger of Berne offers a more suitable solution to this program. On the presupposition that the organic carbon was entirely fossil derived and that the 106 M A Geyh

-12 12000 4

c alcite s aturation C org oxida tion lim e d isso lution

-14 10000

3 8000 -16

]

‰

[ 3 C 6000 1

C δ 3 -18 2

1 d e g a C DIC age (yr B.P.) 4000 IC IC 14 D D -20 14 C DIC concentration (mmol/l) 2000 1 2- -22 2CH2 O + SO4 CO2 + H2 O + CaCO3 - 0 H 2 S +2HCO3 Ca(HCO3 ) 2 -24 0 1234reaction

3 :4 1 P M 9 9 -K ra A u g . 3 1 , 1 9 9 9 8 :1

Figure 4 Schematic diagram of the changes in 14C age, DIC concentration and δ13C values of DIC of groundwater due to oxidation of fossil organic matter and the subsequent dissolution of fossil carbonate (Geyh and Kantor 1998)

DIC concentration was determined precisely, then the age of the DIC could be determined from the product of the measured 14C activity and the DIC concentration viz.,

t (yr BP) = 5730/ln2 × ln (DICo × No)/(DIC × N) . (4) Most recently, Harrington and Herczeg (1999) suggested using the 87Sr/86Sr isotope ratio as the basis for application of a suitable correction to the 14C activity of DIC which had been influenced by input to the groundwater of CO2 formed by the oxidation of fossil organic matter. This involves the application of a two-component mixing model in which the end-members are distinguished by their respective 87Sr/86Sr isotope ratios.

Dissolution of Dolomite Although the dissolution of magnesium subsequent to the calcite saturation is a slow process, it is common under closed conditions in the aquifer. Two carbonate molecules are released into a solution for each Mg2+ ion and consequently result in an increase in DIC concentration. Even in instances where this reaction is buffered by calcite precipitation, the “Oeschger correction” (Equa- tion 4) can be used, whereas the alternative “Gonfiantini correction” may not be applicable (Clark and Fritz 1997).

Admixture of Fossil Carbon Dioxide

The admixture of geogenic or magmatic CO2 (from deep crustal or mantle sources) is often associated with thermal metaphormism and usually accompanied by an increase of the mineralization of the groundwater. Complex hydrochemical reactions may occur. This situation is generally indicated by the occurrence of heavier δ13C values i.e., more positive than approximately –9‰ and DIC concentrations that exceed approximately 8 mMol carbon/liter. The NETPATH program (Plummer et al. 1994) can be used to model the corresponding evolution of the 14C activity and the hydrochemical composition. However, this approach requires that hydrochemical equilibrium has been established, that the possibility of outgassing of CO2 can be excluded (which is seldom the case in practice), and that the flow path of the analyzed groundwater is entirely localized. 14C Analysis of Groundwater 107

Methanogenesis and/or the Admixture of Fossil Methane The process of methanogenesis, which involves the microbacterial degradation of detrital organic matter, in accordance with Equation 5, can occur in both the unsaturated and the saturated zones of aquifer systems.

2CH2O CO2 + CH4 (5) The biochemical pathway is invariably marked by a significant fractionation of the carbon isotopes. Attempts to quantify an appropriate correction for the 14C activity of the DIC are usually unsuccess- 14 ful since the source organic matter is likely to have finite but unknown C activity, and some CO2 is liable to have outgassed from the groundwater prior to carbonate dissolution. The “Oeschger correction” (Equation 4) can be applied, but as discussed previously, only if it may be assumed that the 14 source organic matter is fossil (essentially C free) and that no loss of CO2 gas has occurred. The incorporation of thermocatalytic, abiogenic, or mantle fossil methane is often associated with the admixture of fossil CO2 into the groundwater. Gas will expel after exchange with the dissolved 14 CO2 resulting in an uncontrolled loss of C. In any case, methane is seldom found in resources of fresh groundwater (Geyh and Kuenzl 1981).

PHYSICAL PROCESSES In addition to hydrochemical reactions and advective mixing it is necessary to recognize that other physical processes that can occur within the defined closed aquifer system have the potential to distort the relationship between recorded 14C activity in DIC and straightforward radioactive decay.

Bicarbonate/Matrix Exchange DIC in groundwater diffuses into the pores of the aquifer rock from where it loses 14C by radioactive decay. Subsequent backwards diffusion the 14C depleted DIC will lower the activity of the groundwater flowing through the wide fractures of the aquifer system. Maloszewski and Zuber (1984) showed that a simple relationship exists between the actual age of the water(tactual) and the apparent DIC age (tapp) governed by the matrix porosity (nmatrix) and the fractural porosity (nfrac) of the aquifer rock viz.,

tactual = tapp /(1 + nmatrix /nfrac) . (6)

In carbonate aquifers, the term (1 + nmatrix /nfrac) may be approximated at 2 (Maloszewski and Zuber 1984).

Diffusion Groening and Sonntag (1993) showed the hypothetical influence of a molecular-diffusive penetration of modern carbon into shallow phreatic aquifers obtained in field conditions. This effect resulted in an increase in the 14C activity of DIC. The corresponding decrease, as reflected in the measured 14C age of the DIC, was especially marked in the case of old ascending groundwater. Under such conditions a Pleistocene aged groundwater may signal an apparent Holocene recharge event. This pro- δ13 − cess can be recognized by increased C values, since that of atmospheric CO2 ( 8‰) is heavier than that of DIC at approximately –12‰. Hence, the enrichment of the 14C activity of DIC by the 14 admixture of atmospheric CO2 with its high C activity can be estimated. 108 M A Geyh

APPLIED HYDROGEOLOGY AND THE EFFECTS OF GEOHYDRAULICS In applied hydrogeology the application of isotope hydrological methods is mostly in relation to the management of fresh groundwater resources that have to be exploited for drinking water supply. Therefore, there is usually a high concern over groundwater quality. Secondary hydrochemical reactions are restricted to one or two processes in freshwater systems only, and the induced changes in the isotopic composition can be accounted and corrected. Precise determination of the absolute age of the groundwater is seldom the main criterion. In most cases, it is sufficient to know whether or not the groundwater is being continuously recharged. If water balance modeling is required, then 14C ages of the component DIC may be needed to allow calibration of the model or to improve the hydrogeological conception of the system in context of appropriate analyses. Objections to applied groundwater dating are still voiced among hydrogeologists and isotope hydrol- ogists. For the most part these criticisms tend to be based, quite unjustifiably, on a lack of experience and/or a misconception that the method is unreliable due to 1) the need for a critical evaluation of the initial 14C activity, and 2) the effects of secondary isotopic changes induced via the subsequent hydrochemistry. Indeed, any tendency to apply the various isotopic and hydrochemical correction models to measured and interpret measured 14C activities without a critical evaluation of the characteristic hydrogeochemical features of the natural aquifer system is likely to detract from the objective confidence that can be applied to the reliable 14C dating of groundwater (Mook 1976; Clark and Fritz 1997). Without the necessary attention to selecting the most appropriate correction procedure, the resultant ages calculated for a specific sample of DIC can vary by up to 5000 years (Geyh 1992). Experience gathered in numerous hydrological case studies of freshwater systems shows that secondary hydrochemical induced changes to the 14C activity of DIC are the exception rather than the rule. Where they do occur, correction of the measured data is usually quite simple. The most common and more critical interference for objective dating are apparent shifts caused by the advective mixing of groundwaters of differing ages (e.g. Geyh and Backhaus 1979). However, the prime purpose of multi-environmental isotope analyses is often focused on conceptional modeling in order to yield qualitative or even quantitative hydrodynamic information rather than the determination of groundwater ages per se.

Natural System Conditions Groundwater balance studies have to distinguish between stationary and non-stationary groundwater recharge. Neglecting this aspect in any groundwater budgeting exercise may result in a considerable overestimation of the sustainable resources (Verhagen et al. 1991).

Stationary Recharge Conditions Groundwater flows from the recharge area to the discharge area, and during this passage the 14C age of the component DIC increases. In active aquifer systems the rate of groundwater recharge is bal- anced by the discharge rate and a two-dimensional spatial of the 14C ages of DIC reflects the actual groundwater flow direction and velocity. This information allows the basis for estimation of sustainable yield. In the first instance, 14C dates of DIC are usually interpreted under the assumption that the groundwater flow is analogous with piston-flow conditions and that any admixture within the groundwater mass can be discounted. This assumption may, however, only be valid in respect of confined aquifer systems. In phreatic aquifers, groundwater is recharged over the whole geographical extension 14C Analysis of Groundwater 109 resulting in a vertical component to the 14C activity distribution and which is in turn a function of recharge rates (Vogel 1970). If the aquifer has a constant thickness then there is no spatial distribution of the 14C activity. This means that the 14C age of DIC in groundwater from the same depth is constant from one location to another one. In practice, groundwater supplies from such aquifers are abstracted over a limited vertical range, which is determined by the filter length of the well. The resulting mixture of different aged waters can be described by the exponential model (Verhagen et al. 1991). So-called “conceptional models” have been developed and applied to represent even very complex aquifer systems (e.g. Geyh and Backhaus 1979; Pearson et al. 1983; Wigely et al. 1984; Phillips et al 1989; Verhagen et al. 1991). The regionally valid hydraulic information derived via this approach is often superior to the local information obtained from pumping tests. Tamers et al. (1975) carried out the first applied hydrological study on the exploitation of the deep groundwater of the Biscayne aquifer in southern Florida. The main objective was in support of a pollution prevention assessment. In another case study, Geyh et al. (1984) designed a conceptional model to determine regional hydraulic parameters of the aquitard in a leaky aquifer system north of Nuremberg, Germany, and to establish a water budget (Figure 5). This resulted in a revision of the original hydrogeological conception of the system. It was found that contrary to former ideas, 90% of the groundwater of the deep confined aquifer was in fact being recharged by seepage of shallow groundwater, and that the hydraulic conductivity of the aquitard was of the order of 10−10 m/sec.

14,100 m 8800 m m a.s.l. 400 LEITENBERG FORTH SCHNAITTACH >37,500 yr BP piezometric surface 17,700 yr BP upper aquifer upper aquifer lower aquifer 1.7 m

6.5 m 300 7030 yr BP K = 7.8 x aquitard 10-10 m s-1 K = 3.8x10-10 m s-1 6.0 m K = 3.8x10-10 m s-1 Q =24% v lower aquifer v = 0.9 m yr-1 Q =75% v = 4.2 m yr-1 v = 3.4 m yr-1 v 200 Qv=93%

Figure 5 Hydrogeological section along the direction of groundwater flow in the study area, showing; the conventional 14C ages of the deep groundwater, the regional hydraulic conductivity (m/s) of the aquitard, the percentage of vertical inflow of groundwater (Qv) and the tracer velocity (Geyh et al. 1984)

The immediate challenge in the field of groundwater 14C dating remains with the development of improved procedures that can enable 14C dates measured for DIC to be applied more objectively in the calibration of finite-element numerical mass-transport models.

Non-Stationary Recharge Conditions During the geological past, the hydrological conditions that prevail in modern arid and semi-arid regions oscillated between dry and pluvial periods (Geyh 1992). Furthermore, in present-day humid 110 M A Geyh regions the paleo-hydrological conditions changed during the transition from glacial to interglacial periods and vice versa (Bath et al. 1979). Both scenarios exemplify the existence of non-steady-state recharge conditions during the past history of the particular aquifer system. After any interruption of groundwater recharge the hydraulic gradient of the actual “decaying” fossil groundwater level reflects a superposition of the often negligible amount of recharged groundwater onto that of the fossil relict water body (Burden 1977). The two-dimensional spatial distribution of the 14C dates of such fossil groundwater no longer represents the actual flow velocity (recharge rate) or the flow direction. The age pattern only records the flow conditions that prevailed during former pluvial periods (Figure 6). Consequently, groundwater recharge rates are overestimated if the paleohydrology of present arid and semi-arid regions is not taken into account in numerical flow modeling.

Figure 6 Scheme used to estimate the groundwater tracer velocity for the arid Near East in the past (Geyh 1994). Erratic results are obtained if a tracer velocity is calculated from 14C dates of water recharged during different pluvial periods (e.g. from wells A and D).

This is well exemplified in the results of a comprehensive groundwater survey conducted in 1976 north of Khartoum, Sudan, and east of the Nile River (Verhagen et al. 1991). These data were in obvious conflict with those obtained by numerical geohydraulic modeling of the water budget. A rather simple one-dimensional mass transport model applied to the 14C dates obtained from DIC indicated an actual infiltration rate of only 1.5 to 4.0 × 106 m3/yr along the river bank as opposed to a value of 14C Analysis of Groundwater 111

60 × 106 m3/yr obtained by steady-state modeling—a situation where the actual groundwater availability is an order of magnitude smaller than that estimated erroneously via numerical modeling.

Anthropogenic Disturbance of Hydraulic Systems During the last several decades, there has been a widespread disturbance of the natural balance between recharge and discharge of groundwater resources subject to human exploitation. This situation applies in both humid and arid regions of the world. The abstraction of groundwater has tended to lower the piezometric surface and increase the amount of percolation through aquitards that separate adjacent aquifers. As a result, the 14C ages of DIC often changed without any hydrochemical indications. This apparent mobilization and mixing of groundwaters from different depths and/or between previously discrete aquifer systems provides clear evidence for over exploitation, and in extreme cases, “groundwater mining” (Figure 7; Geyh and Backhaus 1979).

40000 Tiefenthal 35000 Uni BretzS

30000 Wall Hildegartis Göttelmann 25000

20000 C age (yr BP) (yr age C

14 15000

10000

5000

1965 1970 1975 1980 1985 1990 Figure 7 Temporal changes in the conventional 14C ages determined for DIC from groundwater in the heavily exploited Mainz aquifer system

Monitoring of the temporal changes in 14C activity of DIC in such aquifers has highlighted the considerable variation that can occur within a few years, and obviously raises doubts over the reliability of any 14C ages so derived (Geyh 1986). For example, 14C dates have been determined since 1968 from a 200-m-deep unconfined limestone aquifer in Mainz, Germany (Geyh and Sonne 1983). The measured 14C activities show three deviating trends through time (Figure 7) that are significantly devoid of any local clustering. Analogous observations have been made in other unconfined and confined, fractured and sedimentary aquifers (Geyh and Soefner 1989; Verhagen et al. 1991). The only feasible explanation is that sometimes it is deep old groundwater that is mobilized and admixed, and at other times it is young shallow groundwater. A model that allows a quantitative explanation of these patterns has still to be developed. 112 M A Geyh

Exploitation of temporal changes in the conventional 14C dates measured for DIC to quantify the effects caused by groundwater abstraction can only apply where the aquifer system is characterized by simple hydraulic conditions. This was the case for the aquifer in the Azraq spring area described by Verhagen et al. (1991). Here, mixing of young groundwater from the basalt aquifer and old groundwater from the deep limestone aquifer is forced by anthropogenic depression of the water table.

CONCLUSION 14C dating of the DIC in palaeowater is indispensable for paleohydrological and palaeoclimatologi- cal reconstructions and for applied hydrogeological studies. Numerical mass-transport modeling profits from the improvement of hydrogeological conceptions and the revision of boundary conditions. In the case of non-steady-state recharge conditions, maximum and minimum recharge rates in the past may be estimated from 14C dates of DIC via numerical modeling. 14C dates measured from DIC enable the determination of regional geohydraulic parameters and allow water balances to be checked. In most applications, complementary environmental isotope analyses of hydrogen and oxygen together with general hydrochemical measurements are recommended to facilitate a comprehensive and thorough interpretation of the 14C dates (Cark and Fritz 1997). A primary task towards a significant improvement in the applied 14C dating of groundwater remains with the development of procedures that will enable 14C activities measured from DIC to be confi- dently included in the calibration of finite-element and compartmental modeling exercises.

ACKNOWLEDGMENTS Renee Kra encouraged the submission of papers describing the 14C dating of groundwater, although this topic was far removed from her own professional expertise. Her constant interest and support during her involvement with the editorial office of Radiocarbon has been greatly appreciated. I am particularly grateful of her patient assistance with critical reading of my German-English mixed manuscripts. This editorial support often helped my science far more than additional costly analyses could have.

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EVOLUTION AND MULTIDISCIPLINARY FRONTIERS OF 14C AEROSOL SCIENCE1

L A Currie Fellow, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8370, Gaithersburg, Maryland 20899-8370 USA. Email: [email protected].

ABSTRACT. A review is given of some critical events in the development of radiocarbon aerosol science, and the profound influence of radiocarbon accelerator mass spectrometry (AMS) on its current applications and future prospects. The birth of this discipline occurred shortly after the initial development of 14C dating. Unlike dating, which is founded on the continual decay of 14C and the resulting full range of 14C/12C ratios in once-living matter, 14C applications to atmospheric aerosol research relate primarily to the determination of mixing ratios of fossil and biomass components. Such determinations have come to have major importance in work ranging from the resolution of woodburning and motor vehicle components of urban particulate pollution, to the apportionment of radiation-forcing (black) particulate carbon from natural wildfires and anthropogenic regional plumes. The development of this area has paralleled that of AMS itself, with the one or the other alternately serving as the driving force, in a sort of counterpoint. The remarkable million-fold improvement in sensitivity made possible by AMS has become critical in meeting rapidly emerging societal concerns with the origins and effects of individual carbonaceous species on health and climate.

INTRODUCTION Shortly after Libby’s initial work on radiocarbon dating, Jim Arnold, and later Hans Suess, had the idea to apply natural 14C to the apportionment of fossil and biomass combustion aerosol (“soot”) which was afflicting several US cities. The first such experiment by Clayton, Arnold, and Patty (Clayton et al. 1955) took place just seven years after Libby’s basic experiments that led to 14C dating. The stimulus was the need to determine the fossil carbon impact on serious levels of particulate urban pollution, equivalent to 35 µg C m−3 in Pasadena, California at the time. The sampling efforts were heroic: nearly a week of continuous sampling was required to filter about a quarter of a million cubic meters of air, collecting approximately 42 g of aerosol to provide 8.5 g of carbon for measurement by liquid scintillation counting. The results foreshadowed what has been seen in many subsequent urban studies: the aerosol carbon was primarily, but not exclusively, from fossil sources (fraction of modern, fM = 0.26 ± 0.02). (Note that all uncertainties are combined standard uncertainties, unless otherwise indicated.) Work performed five years later by Lodge, Bien, and Suess (Lodge et al. 1960) had a similar grand scale: 3.8 g aerosol carbon were collected and decay counted with acetylene gas proportional counting. This research went beyond the earlier study, in that individual chemical fractions were measured; it was the first example of aerosol 14C speciation. Results were similar, however, showing that the urban carbonaceous particles were largely fossil in origin. More recent work in US cities has given some insight into current sources of the non-fossil (biomass carbon) component, ascribing a significant portion to commercial “meat cooking” (Hildemann et al. 1994) or, during winter, to residential wood combustion (Cooper et al. 1981). Progress in the ensuing years has been enormous, primarily for 2 reasons: 1) gains in sensitivity, first with the development of miniature gas proportional counters for 14C in aerosol carbon at the µg level, followed by accelerator mass spectrometry (AMS) with an intrinsic mg level capability, and 2) increased concern about the health consequences of carbonaceous aerosols, their impact on visibility, and more recently, the potential effects on climate from the radiation forcing behavior of “black carbon” or soot arising from fossil and biomass fuel combustion. For all of these reasons it becomes

1Contribution of the National Institute of Standards and Technology; not subject to copyright.

115 116 L A Currie important to identify sources and apportion aerosol carbon, even at the molecular level (individual compounds). This gives rise to a new dimension in 14C atmospheric and geochemistry, where we seek to determine the spatial and temporal distribution of 14C in individual compounds and chemical fractions— i.e. 14C speciation. This lay, of course, at one of the frontiers of modern AMS.

EARLY “LOW-LEVEL” ATMOSPHERIC STUDIES After a lapse of about two decades, 14C measurements were applied anew to atmospheric aerosol research, this time in the US cities of Los Angeles and Salt Lake City, and in the Utah desert. There had been a dramatic reduction in sample size, however. In this case only 5–10 mg of carbon was required, as the measurement was performed with miniature (5 mL) high purity quartz gas proportional counters. (See Figure 2 for low-level counting [llc] information, and Currie et al. [1983, 1998a] for characteristics of the miniature gas counting system. Note that a very recent change to “active” anticoincidence shielding in our laboratory has reduced the background by about a factor of 3 [G Klouda, personal communication 1998], such that the limiting factor has become Poisson counting statistics.) The thousand-fold gain in sensitivity marked the turning point for 14C aerosol research, in that daily and even diurnal sampling of fine particle (respirable) carbonaceous aerosol became practicable. Results for the urban samples were similar to those of Pasadena (approximately 60–70% fossil carbon), but the desert aerosol was essentially “living” (only 10% fossil carbon). Interestingly, these data were reported at the very first AMS conference, AMS-1, in Rochester, New York (Currie 1978). During the next decade important urban results were obtained with both low-level gas counting of 5–10 mg (C) samples and AMS on 0.1–2. mg (C) samples. Noteworthy examples of the former were the first multivariate time series field studies, which took place in Oregon (Cooper et al. 1981) and in Norway (Ramdahl et al. 1984), where simultaneous data on 14C and chemical species were obtained on a series of fine particle (<2.5 µm) aerosol samples for the purpose of apportioning multiple sources of combustion aerosol. The multivariate time series character of the data provided the key to further dissecting (discriminating, apportioning) the fossil and biomass source contributions into individual source components, such as motor vehicle emissions, fuel oil combustion, and field (grass) and slash (timber) burning, through such techniques as chemical mass balance and factor analysis (Currie 1992). To illustrate, the latter study took place in the small town of Elverum, Norway (approximately 10,000 inhabitants), where average wintertime aerosol carbon and polycyclic aromatic hydrocarbon (PAH) pollution levels of 20 µg m−3 and 158 ng m−3, respectively, were comparable to those of Oslo (approximately 400,000 inhabitants), presumably due in large part to the use of wood fuel as a secondary heat source. This was the first study to combine time series data for 14C, chemical elements, PAH, mutagenicity, and meteorology. It showed that, on average, 65% of the aerosol carbon came from woodburning. This work led also to the development of a multivariate isotopic-chemical “urban signature” allowing the discrimination of long-range transport aerosol from the urban background aerosol. This is shown in Figure 1 (see Color Plate 1), which represents an optimal (principal component) two-dimensional projection for the subset of samples identified as long range transport [sample-10] or as belonging to the urban cluster (Currie 1992). It is seen that the two-dimensional representation gives a reasonably complete view of the entire seven-dimensional variable space, since it preserves most of the original information (95% of the variance). The multivariate (approximately bivariate) urban signature is indicated by the elongated, dashed ellipse. As shown by the Pb and 14C projections on the principal component biplot, the signature is approximately bi-polar, reflecting the varying mix of motor vehicle (Pb) and woodburning (14C) contributions to the urban 14C Aerosol Science 117 aerosol. The pattern for the long range transport aerosol sample [sample-10], having tracers found in coal fly ash, is almost totally resolved from that of the urban aerosol samples. Note that the multivariate signatures represent a powerful generalization of the “element ratio” technique that has been applied to identify source regions in cases of long range pollution (Rahn and McCaffrey 1980). It is expected that the multivariate signature developed in this work will serve as a prototype for future investigations of regional and global aerosol where such signatures may be used for the unique identification of local and regional aerosol sources. A critical example relates to the identification of source regions for remote aerosol, such as the “black carbon” transported to polar regions, as discussed in the penultimate section of this article. The most extensive, early 14C aerosol study utilizing AMS was the Integrated Air Cancer Project (IACP), organized by the US Environmental Protection Agency (EPA) (Lewtas et al. 1988). This was a multi-city investigation of aerosol mutagenic activity as related to motor vehicle and residential woodburning sources. Notable AMS related outcomes included one of the first examples of 14C speciation in urban ambient aerosol (14C in elemental carbon [EC] and polycyclic aromatic hydrocarbon [PAH] chemical fractions), and the impact of the bivariate (12C, 14C) chemical processing blank on the validity of some of the aerosol data. (See the discussion in the following section.) The IACP involved diurnal sampling (07.00–19.00, 19.00–07.00) during approximately two months in the winter in Raleigh, North Carolina and Albuquerque, New Mexico (1984–1985), Boise, Idaho (1985–1986), and Roanoke, Virginia (1986–1987). Typical sample sizes for AMS were 300–500 µg carbon. For a subset of samples 14C speciation was applied, showing the elemental carbon fraction to be generally more fossil in character (41%) than the total extracted organic matter (18%) (midranges, Albuquerque; Klouda et al. 1988). The combination of 14C and mutagenicity testing on the extracted organic matter showed that, on average, the concentration of carbonaceous aerosol from woodburning (16 µg C m−3) was approximately twice that from motor vehicles, but that the mutagenic potency of the latter (3.4 rev mg−1 aerosol) was greater by about a factor of 4 (Currie et al. 1989, and references therein). Using PM10 particulate matter samples collected in the Boise phase of the IACP, Benner and coworkers demonstrated the ability of dimethylphenanthrene iso- mers to distinguish residential wood combustion from mobile source emissions; excellent correlation was obtained with 14C data derived from the same field samples (Benner et al. 1995).

THE TRANSITION FROM MG-LEVEL BETA COUNTING (AND AMS) TO µg-LEVEL AMS A comparative study of the fundamental limiting factors for AMS and low-level β counting for environmental 14C research showed that: 1) for AMS, the isotopic-chemical blank constitutes the most important limitation, outweighing the machine background by 1 to 2 orders of magnitude, but 2) for β counting, the situation is reversed, with the typical blank being quite negligible compared to the counter background (Currie et al. 1989). (See Figure 2, where limits are expressed in terms of modern carbon equivalent mass.) The study showed also that the nominal AMS sample size of 1 mg C was 100–1000 times larger than the blank, so that the prospect of “µg AMS” for environmental 14C was quite real, provided that the overall blank could be brought under control. This matter, control of the isotopic-chemical blank for aerosol 14C research at the microgram level, has been one of the foci of the Atmospheric Chemistry Group at NIST, in cooperation with colleagues at the AMS facilities at the University of Arizona (NSF-Arizona AMS Facility), the Woods Hole Oceanographic Institution (National Ocean Sciences AMS Facility), and the University of Vienna (Vienna Environmental Research Accelerator). The potential benefit of “µg AMS” is enormous, because the resulting additional thousand-fold reduction in sample size is critical for expand- ing the frontiers of atmospheric aerosol research to the assay of 14C in EC and individual organic 118 L A Currie tracers and toxic compounds, such as certain of the PAH. The capability is vital for research on the regional transport and historical (ice core) record of fossil and biomass combustion aerosol in envi- ronmentally sensitive regions, such as the Arctic, where EC concentrations in snow/ice may be as little as 1 µg kg−1. The inherent AMS capabilities and target preparation blanks resulting from NIST collaborative work with the previously mentioned AMS facilities have been reported at previous international AMS and 14C conferences based on 14C measurements using the NIST Fe-C “bead” type target (Verkouteren et al. 1987; Klinedinst et al. 1994; Weissenbök et al. 1998). Although it has been possible to constrain chemical blanks to approximately 2 µg carbon or less, there have been difficulties with 12C− ion current magnitude and stability when the total carbon content of the bead dropped below approximately 10 µg carbon. This has been a special problem when vigorous chemical processing (e.g. for isolating elemental carbon) may have introduced trace levels of electronegative impurities. Other laboratories, utilizing “graphite” targets, have been generally successful in measuring samples containing less than 100 µg carbon, but 20 µg is the typical lower limit (Vogel et al. 1987, 1989; Pearson et al. 1998). More recent efforts to overcome the 10 µg barrier using “dilution AMS” with on-line purity monitoring appear to be successful for pre-combustion samples containing as little as 1–2 µg carbon (Currie et al. 2000a).

Figure 2 For aerosol research the minimum amount of modern carbon required for AMS measurement is about 3 orders of magnitude smaller than that needed for miniature counter decay counting (llc) (Currie et al. 1989). The switch in the fundamental limiting factor, from detector background to the isotopic-chemical blank, has a profound effect. Although a blank of approximately 2 µg carbon can be achieved for the combustion-target preparation process, wide-ranging bivariate blanks may arise in aerosol sampling and sample preparation steps in a complicated field study, as shown in the plot. The points within the ellipse were generated for Monte Carlo error propagation, to simulate the actual distribution of blanks seen in one phase of the Integrated Air Cancer Project (Lewtas et al. 1988). 14C Aerosol Science 119

The isotopic-chemical blank for the overall (aerosol) measurement process is another matter. One must pay strict attention to the fossil and biomass carbon blanks introduced at each step, from aerosol sampling to chemical extraction and purification. The impact of this larger blank issue is indicated in the plot (inset) in Figure 2. This shows the covariation of fM and carbon mass over a wide range for organic extraction blanks during one phase (Roanoke, Virginia) of the Integrated Air Can- cer Project, where the typical blank and total sample carbon masses were approximately 70 µg and 400 µg, respectively. Although the processing blank was subsequently much improved, this large bivariate blank had an enormous impact on an entire set of Roanoke aerosol samples, especially with respect to the magnitude and nature of the final uncertainty bounds. Not only must the correlated behavior be taken into account in blank “error propagation”, but also the non-normality introduced by the non-linear blank correction function. This leads to poorly conditioned and asymmetric uncertainty intervals when the blank mass fraction becomes too large (Currie et al. 1994).

DUAL ISOTOPIC CHARACTERIZATION: APPORTIONMENT AND FRACTIONATION ISSUES An additional degree of source discrimination/apportionment capability for atmospheric aerosols is given when both 13C and 14C are utilized. The two isotope ratios that characterize the 13C-14C plane make possible the resolution of up to three sources or end members, as in Figure 3. If four or more end members are likely, the solution is indeterminate. For example, if C4 plants, such as sugar cane or tropical grasses, had also been possible sources, it would have been necessary to include a fourth biomass 13C value of about −10‰ (Cachier 1989). For an illustration of the 13C-14C signatures of a broad range carbon isotopic reference materials, see Figure 16 in Currie (1992). The particular example given in Figure 3 is that of the prototypical NIST Urban Particle Standard Reference Material (SRM 1649a) [U] that was originally collected in Washington, DC (1976–1977) to serve as an aerosol organic chemical standard. This material is becoming increasingly used, also, as a carbon isotopic reference material by atmospheric and marine scientists. Of special interest, with the advent of GC/AMS and other 14C speciation techniques, is the utilization of the SRM in research on the isotopic composition of individual compounds and classes of compounds (see also Figure 4 in Color Plate 1). A second SRM serves as one of the end member reference points. The point marked [D] in the plot indicates the 13C-14C composition of the diesel “soot” standard reference material, SRM 1650. (Note that the apportionment of “U” into pine, oak, and diesel end members is intended to illustrate the principle of dual isotopic source apportionment. It should not be taken literally for this specific material, for which there could be a host of fossil and biomass carbon sources in the Washington, DC urban area.) A special issue related to Figure 3 is the isotopic and chemical fractionation that accompanies incomplete combustion. That is the reason for the comment about the extra 13C dispersion for the oak and pine end members. A report of a detailed laboratory combustion study of this phenomenon has recently appeared (Currie et al. 1999a). In contrast, isotopic fractionation associated with pho- tosynthesis is enzyme catalyzed. For C3 plants (Calvin-Benson cycle), the first stable product following carboxylation with the enzyme ribulose-1,5-diphosphate carboxylase is a three-carbon compound; for C4 plants (Hatch-Slack cycle), the first stable product following carboxylation with the enzyme phosphoenol-pyruvate carboxylase is a four-carbon acid (Deines 1980). Dual isotopic characterization of carbonaceous materials has recently taken on another role of practical, economic importance—namely the 13C-14C “fingerprinting” of industrial feedstocks and commercial products. A case in point is the authentication of a new industrial copolymer derived from a particular class of biomass feedstocks, where its compositional uniqueness is established by its loca- 120 L A Currie

Figure 3 Plot showing the apportionment of the total carbon of urban dust standard reference material, SRM 1649a [U] into diesel [D], Pine and Oak end members. This figure contrasts with Figure 1 in that the goal here is end member apportionment rather than discrimination. The extra 13C dispersion noted for the pine and oak members yields an added uncertainty component to the apportionment estimates. tion in the 13C-14C plane (Currie et al. 2000b). One consequence of such applications, having significant legal and economic ramifications, is the critical need for a range of carbon isotope reference materials that span the relevant two-dimensional “fingerprint” regions to provide defensible quality assurance for dual isotopic measurements of such industrial materials.

14C SPECIATION; AND A SPECIAL ATMOSPHERIC REFERENCE MATERIAL Figure 4 shows two manifestations of the urban particle standard reference material (SRM 1649a) which was prepared specifically as a quality assurance material for the measurement of organic species in atmospheric aerosol samples, and which in recent years has been characterized for carbon isotopes in selected chemical fractions (NIST 2000). The SRM, per se, has been available for many years in bulk form (rear of photo). Within the last few years prototype aerosol filter samples (front of photo) have been prepared by redistributing the bulk SRM on 47 mm quartz and polycarbonate aerosol filters, to serve ultimately as carbonaceous particle filter reference samples (Klouda et al. 1996). This is especially important for aerosol carbon measurements that are based on optical and thermal-optical methods (Huntzicker et al. 1982). It is the norm for ambient atmospheric aerosol to be isotopically heterogeneous—i.e. to have different 14C/12C ratios for different chemical fractions. This reflects, of course, the differing chemical compositions of aerosol from fossil and biomass combustion (or emission) sources. The extent of 14C Aerosol Science 121 the 14C isotopic diversity of SRM 1649a is shown in Figure 4 both for classes of organic species and for individual compounds. There, it is seen that the aliphatic components are derived almost entirely from fossil sources, presumably in this case (Washington, DC) motor vehicle emissions, whereas the polar organic fraction has a rather significant biomass contribution, such as might be derived from the incomplete combustion of wood or other biopolymers. In keeping with the focus on fossil-biomass apportionment of carbonaceous aerosol, the data in Fig- ure 4 are presented in terms of the (living) biomass carbon percentage composition. The fossil carbon composition is then complementary. If interest centers on a biomass component that represents a range of years, such as woodburning soot, adjustments may be made for its lifespan (Currie et al. 1989). The SRM reference values (not shown), however, are expressed as fraction (or percentage) of modern carbon, fM. Two corrections are needed to convert from fraction of modern to fraction of 14 (living) biomass: 1) correction for C decay from sampling date to fM reference date, and 2) correction for the time-dependent enhancement of biospheric 14C due to atmospheric nuclear testing. The first correction is generally quite small, given the 5730-yr physical half-life; the second can be sub- 14 12 stantial, since the C/ C ratio in atmospheric CO2 approximately doubled during the mid-1960s. For SRM 1649a fM values referenced to 1998, with an average sampling date of 1976.5, the correction is approximately (1.003/1.35)=0.743—i.e. the biomass carbon fraction (referenced to the date 14 of sampling) equals 0.743 times fM (referenced to 1998). For example, new data for total carbon C in SRM 1649a (NIST 2000; Currie et al. 1999b), show an average fM (1998) value of 0.509, so the corresponding biomass fraction is (0.509)(0.743)=0.378.2 Two matters deserve special noting: 1) “Elemental carbon” is one of the most important species in atmospheric aerosol science, yet it is one of the most problematic from the perspective of metrology; hence, the parenthetical result given in Figure 4. There are major interlaboratory comparisons (chemical and isotopic) underway at present in efforts to reach consensus on this matter (Currie et al. 1999b). The value quoted comes one such interlaboratory effort involving the University of Cal- ifornia Irvine (C Masiello), NIST (L Currie et al.), and the University of Arizona (D Donahue et al.), for which fM equals 0.153 ± 0.002 (Poisson standard uncertainty), and 2) The results for fluoran- thene and benzo(ghi)perylene are also very recent data, linked to baseline resolved GC/AMS (Currie et al. 1999a). A more extensive view of the latter is given in Figure 5, which gives a comparison of the multivariate statistical route for individual compound “dating” to direct, pure compound isolation for AMS 14C measurement, in this case by “off-line” GC/AMS (Eglinton et al. 1996; Currie et al. 1997). Figure 5 gives further insight into 14C speciation in aerosol PAH. The diagram at the left comes from a multivariate urban field study that showed benzo(ghi)perylene to be highly correlated with the motor vehicle (fossil emissions) tracer Pb, with a quantitative estimate of 92% fossil carbon, based on multiple regression (MR). The panel at the right of the figure shows six PAH fractions isolated from the SRM by preparative capillary gas chromatography for 14C measurement by AMS. The result immediately below this panel shows the benzo(ghi)perylene, again, to be primarily fossil. Both the MR result and this one suffered from substantial uncertainties, however; the former, because of the limited number of samples and the error amplification (variance inflation) that typi- fies most multivariate source resolution studies; the latter, because of imperfect isolation of the PAH

2Recent information indicates that SRM 1649a was sampled during 1976–1977 rather than 1973, the assumed sampling date 14 12 for SRM 1649a in Currie et al. (1997, 1999a). Because the C/ C ratio in atmospheric CO2 was about 6% higher in 1973, the computed biomass carbon fractions in these references should be increased by a factor of 1.06 (Note that at the time of 14 these earlier publications, only the early, relatively imprecise total C value, fM=0.61, was available.) 122 L A Currie from the underlying “unresolved complex mixture” (ucm) seen in the bottom chromatographic trace of the material in the waste trap. The baseline resolved result at the bottom right of Figure 5 (94 ± 1% fossil), is a very recent datum that was achieved by the offline GC/AMS technique where the influence of the baseline was essentially eliminated through the special pre-purification technique of PAH ring size fractionation (Wise et al. 1977; Pearson et al. 1997). Two very important conclusions derive from this work. First, compound-specific techniques such as on-line GC/AMS cannot give high quality results unless the individual compounds meet the baseline resolved criterion; unresolved material that is co-eluted is directly analogous to the isotopic-chemical process blank discussed earlier in connection with Inte- grated Air Cancer Project. Second, the baseline resolved result for benzo(ghi)perylene shows that this compound, which has been broadly accepted as unique tracer for fossil fuel combustion aerosol, has a small but definite biomass carbon component in this urban particle SRM. The latter conclusion is consistent with the observed production of this compound during the flaming phase of the laboratory combustion of pine (but not oak; Currie et al. 1999a).

EMERGENCE OF 14C SPECIATED, NATURAL-MATRIX REFERENCE MATERIALS The Urban Dust reference material discussed in the preceding section foreshadows a major, new trend in 14C reference materials, as the need for procedural quality assurance and isotopic-chemical traceability expands to new horizons. This has been brought about, in part, by the importance of isotopic speciation in a host of scientific disciplines and environmental compartments. One of the driving forces, for example, is the importance of quantitative apportionment of the incomplete combustion tracers, “black carbon” and PAH in the bio- and geosciences (atmospheric, soil, marine, cryospheric,...) (Currie et al. 1999b).3 An equally important driving force is the emerging capability of “dating” extremely small (µgC) samples and especially the “chemical” AMS revolution that holds the possibility of on-line isotopic-chemical characterization of individual compounds, using techniques such as GC/AMS (Shibata 1999). Already, 13C and 14C speciated reference data are being generated for representative sediment materials, for the benefit of the marine chemistry community (Masiello et al. 1999). The solid foundation for this new trend toward 13C and 14C speciated reference materials, in appropriate natural matrices, has been established through several, very important international exercises using 14C reference materials specially selected to be representative of the types actually radiocarbon dated, such as wood, peat, cellulose, limestone, etc. (Scott et al. 1998). Reference materials growing out of these intercomparison exercises have taken their place beside the basic 14C dating oxalic acid standards (SRMs 4990B and 4990C). Lessons learned from these earlier exercises showed also the extreme importance of providing adequate supplies of homogeneous and representative, natural matrix reference materials. As notedin the discussion accompanying Figure 3, dual isotopic characterization of such materials serves the critical quality assurance function of “qualify- ing” the 13C-14C plane in the context of authentication of industrial and commercial products (Currie et al. 2000a).

3Chemical as well as isotopic-chemical speciation has considerable importance in biogeochemical applications. The “black carbon,” for example is of great concern because of its radiation scattering and absorption properties (visibility, climate); and detailed chemical characteristics of the PAH have important implications for biological effects and for inferences concerning petrogenic vs. pyrogenic origins. The breadth of concern for suitable, multidisciplinary reference materials expressed at the Black Carbon Symposium led to the formation of the International Steering Committee for the Develop- ment of Black Carbon Reference Materials, with an early mission to identify and characterize (chemically, isotopically) natural matrix reference materials that are suitable for the atmospheric, soil, and marine sciences (Black Carbon 1999). 14C Aerosol Science 123

Figure 5 Fossil/biomass carbon apportionment at the molecular level (benzo(ghi)perylene). This figure contrasts multivariate statistical vs. direct (GC/AMS) modes for “dating” trace organic compounds in atmospheric aerosol, and shows the sensitivity of the latter to the “ucm” blank (Currie et al. 1997). The diagram on the left, derived from a large field study with day (o) and night (•) sampling, indicates a strong association between fossil motor vehicle emissions (with Pb as surrogate) and benzo(ghi)perylene. The numerical result below comes from multiple regression with Pb and K as regressors. The diagram on the right shows the isolation of several PAH by preparative scale capillary gas chromatography, which preceded AMS on individual molecular fractions of the SRM 1649a. A principal observation is that the initial 14C result for benzo(ghi)perylene has a large uncertainty, due to the large “unresolved complex mixture” (ucm) of unknown isotopic composition that coeluted and was recovered in trap-6, the B(ghi)P trap. The vastly improved result showing 94 ± 1% fossil carbon was achieved through an improved purification technique involving the isolation of PAH ring size fractions (Wise et al. 1977; Pearson et al. 1997; Currie et al. 1999a).

A MULTIDISCIPLINARY CASE STUDY; IMPLICATIONS FOR FUTURE RESEARCH We conclude with highlights from research presented in part at the 1997 International 14C Confer- ence in Groningen (Currie et al. 1998b). This work extends the multivariate aspect of modern 14C aerosol research to multidisciplinary science. In this case the objective was to study the long range transport of dust and combustion aerosol from boreal wildfires to remote regions such as Greenland. The study captures central features of large-scale atmospheric contaminant investigations, ranging from source region identification, to long range transport, to aerosol arrival and deposition at receptor sites. Key elements include: fire count data, transport “observations” via UV and IR satellite imagery plus air mass trajectory analysis, and receptor site sampling followed by multicomponent chemical and isotopic analysis—with a focus on elemental or “black carbon” (EC), because of its radiative properties. As indicated in Figure 6 (see Color Plate 2), information drawn from these several disciplines showed conclusively the causal relation between the boreal wildfires in central Can- 124 L A Currie ada and the singular enhancement of biomass burning aerosol carbon at Summit, Greenland on 5 August 1994. The next direction in the 14C aerosol research is focused on the recovery of the history of aerosol carbon, as captured in polar snow and ice cores. The more recent history is linked to the excavation of samples from snowpits, as illustrated in the photograph from Summit, Greenland (Figure 7 in Color Plate 2). Besides time series data on isotopic and chemical tracers of fire, such as elemental carbon and the PAH, we have the opportunity to investigate individual particles, using extremely powerful microanalysis tools. One such example is the woodburning char particle shown in Figure 7. This was recovered from a depth of 114 cm in a 1996 snowpit, in the vicinity of a 1994 fire horizon (Currie et al. 1998b). The future of such research is extremely promising, having the potential to improve our understanding of: 1) the processes involved in the transport of carbonaceous aerosol and its incorporation in snow crystals and eventually ice cores, and 2) the history of fossil and biomass burning as recorded in atmospheric aerosols trapped in the ice cores.

CONCLUSION AND OUTLOOK The advances in AMS that are making possible the measurement of 14C at the microgram level, and the link with chemical science making possible compound-specific and continuous flow 13C and 14C measurements will enormously enhance our ability to expand isotopic carbonaceous aerosol research to regimes that are remote in time and space. In the Greenland snow, for example, the concentrations of elemental carbon are commonly in the neighborhood of 1 µg per kg of snow, so the improved sensitivity is vital. Among the special challenges are low-level 14C speciation in chemical tracers of fire (e.g. PAH and EC), and improved understanding of isotopic and molecular fractionation in incomplete combustion, including some surprising differences between laboratory and field experiments (Currie et al. 1999a). Also, as the quantity of sampled material gets larger, and the 14C detection limit, smaller, increased effort will be needed to understand and control the chemical and isotopic blank. Similarly, the full benefit of on-line GC/AMS will require creative approaches to achieve baseline resolved peaks. An accompanying challenge, as samples become smaller, and applications, increasingly multidisciplinary, is the provision of appropriate, natural matrix isotopic-chemical reference materials. Finally, with expanded AMS capabilities and highly interdisciplinary approaches, we see the local and urban 14C aerosol researches of the past taking on a regional and even global character, allowing us to better understand the impact of carbonaceous aerosol sources on our planet.

ACKNOWLEDGMENTS This paper was adapted, with permission, from an invited talk (“Recent History and Future Chal- lenges of 14C Aerosol Research”) documented in the Proceedings of the International Workshop on Frontiers in Accelerator Mass Spectrometry, Tsukuba and Sakura, Japan (6–8 January 1999; Shibata 1999). Contributions to the work reviewed have involved a great many scientists and institutions, as well as a number of cooperating and supporting organizations, including: the Climate Change Center (University of New Hampshire); the US Environmental Protection Agency, National Science Foun- dation, and National Aeronautics and Space Administration; and the Norwegian Institute of Air Research. Central to much of the work have been colleagues and guest researchers at NIST; in particular, members of the Atmospheric Chemistry Group. Special acknowledgement for essential AMS collaborations must be given to the University of Arizona (NSF-Arizona AMS Facility), the Woods Hole Oceanographic Institution (National Ocean Sciences AMS Facility), and the University of Vienna (Vienna Environmental Research Accelerator). Credits for figures are as follows: Figure 1: 14C Aerosol Science 125 principal components biplot, derived from cluster and biplot data in §3.4.2 of Currie (1992); Figure 4: photo of SRM 1649a filter reference samples, courtesy of George Klouda, NIST; Figure 5: adapted from Figures 6–7 of Currie et al. (1997); Figures 6–7: (snowpit photo) adapted from Figures 1–2 of Currie et al. (1998b); Figure 7: (SEM image, X-ray spectrum) courtesy of John Kessler, NIST.

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SOME COMMENTS ON ACCELERATOR MASS SPECTROMETRY

Harry E Gove Professor Emeritus of Physics and Astronomy, University of Rochester, Campus Station, New York 14627, USA and Adjunct Professor of Physics, IsoTrace Laboratory, University of Toronto, 60 George St., Toronto, ON, M5S 1A7, Canada. Email: [email protected].

ABSTRACT. This paper discusses some aspects of the development of accelerator mass spectrometry (AMS), the international conferences that have been held, and the books that have been written on the subject. It also mentions some details of the technique and its strengths. Some of the interesting measurements that have been made recently are covered, and finally, it presents some thoughts on future developments.

THE DEVELOPMENT OF ACCELERATOR MASS SPECTROMETRY Accelerator mass spectrometry (AMS) was originally developed to detect radiocarbon in samples many times smaller than required by the Libby decay counting technique and, it was hoped, in organic samples that “died” much longer ago than the 60,000-year or so age limitation of the Libby method. Almost from its inception AMS involved the use of tandem electrostatic accelerators normally employed for research in nuclear physics and, later, small tandems specifically designed to detect long-lived radioisotopes. Some early work used cyclotrons, but at present and probably well into the future, tandem electrostatic accelerators are the ones of choice. The use of tandem electrostatic accelerators for the direct detection of 14C in natural organic samples is particularly advantageous because it allows the discrimination against the two mass-14 interfer- 14 12 13 14 ences. These are N and the mass 14 molecules CH2 and CH. In the case of N, for many years people using tandem accelerators for nuclear research, when they wanted beams of 14N, tuned the negative ion source to 14NH− rather than 14N−. It was assumed that the latter negative ions were sufficiently unstable as to not allow them to be accelerated. Who, among tandem users, actually first discovered they were unstable is open to debate. It was, however, later definitively demonstrated to be the case (Purser et al. 1977). Remarkably enough, in searching the literature for another purpose, I came upon a paper by Sir John J Thompson published in Philosophical Magazine (1912:209). In this paper Thompson, who won the Nobel Prize for physics in 1906 for his discovery of the electron, reported the results of experiments in which he deflected positively and negatively charged atoms in electric and magnetic fields. He noted:

I have never observed any indications of negatively charged helium, argon, nitrogen or mercury, however strong the lines corresponding to the positively charged atoms may have been. On the other hand, the atoms of hydrogen, carbon, oxygen, sulphur, chlorine are conspicuous for the readiness with which under ordinary conditions they acquire a negative charge. Clearly, from the point of view of AMS, Sir John was, albeit unwittingly, well before his time!

12 13 In the case of the mass-14 interfering molecules CH2 and CH, one of the great advantages of AMS employing tandem electrostatic accelerators is the need to convert the negative ions to positive ions in the terminal for the second acceleration. In this conversion, if three or more electrons are removed from the molecule in addition to the electron that made it a negative ion, it disintegrates by a coulomb explosion and the resulting fragments are readily separated from the wanted 14C by electric and magnetic deflection at the high energy end of the accelerator. The destruction of molecules is a property of AMS employing tandem electrostatic accelerators that is of crucial importance in the ultrasensitive detection of every isotope in addition to 14C for which AMS is used.

127 128 H E Gove

Although at the beginning the impetus was to detect 14C, it was soon realized that many other radioisotopes and even stable isotopes could be usefully detected by AMS in the matrices of more abundant elements. In fact, as soon as it is demonstrated that AMS can measure an isotope at high sensitivity for a particular application, many other applications for measurements of that isotope soon blossom. The radioisotopes detected by AMS to date include 10Be, 14C, 32Si, 26Al, 36Cl, 41Ca, 99Tc, 129I, 210Pb, 210Po, 226Ra, 230Th, 231Pa, 234U, 236U, 237Np, 237Pu, and 244Pu. Of these, the ones measured most often are 10Be, 14C, 26Al, 36Cl and 129I. Stable isotopes include Ru, Cs, Rh, Os, Ir, Pt, and Au. Fields in which AMS has made important contributions include hydrology, geoscience, materials science, biomedicine, sedimentology, dendrochronology, archaeology, environmental sciences, oceanography, paleoclimatology, and several others. Many review articles have been written on the science of AMS and its applications since its inception in 1977 (see for example Litherland 1980, 1984; Gove 1985; Elmore and Phillips 1987). The historical development of AMS as it currently exists began in 1977. It has been covered in detail in many review articles (see for example Gove 1992, 1999).

International Conferences on Accelerator Mass Spectrometry The first international conference on AMS was held at the University of Rochester in 1978. It was titled “Radiocarbon Dating with Accelerators” (Gove 1978) reflecting the above-mentioned impetus for the development of AMS. At that meeting, however, papers on the detection of 10Be and 36Cl were also presented. The title of the meeting was subsequently changed to reflect the wider applications of AMS. In 1981 the Second International Symposium on Accelerator Mass Spectrometry was held at the Argonne National Laboratory (Kutschera 1981). The third meeting took place at Zurich in 1984 (Woelfli et al. 1984), the fourth (which celebrated the Anno Decimo of AMS) was held at Niagara-on-the-Lake, Canada in 1987 (Gove et al. 1987), the fifth in Paris in 1990 (Yiou and Rais- beck 1990), the sixth in Australia in 1993 (Fifield et al. 1994) and the seventh in Tucson, Arizona in 1996 (Jull et al. 1996). The eighth was held in Vienna, Austria in 1999. Except for the first two AMS conferences, the proceedings have been published by North-Holland Publishing Division in Nuclear Instruments and Methods in Physics Research. The 14C community has held an International Radiocarbon Conference every three years or so, with the 10th held in Bern-Heidelberg in 1979. That was the first of these meetings to include a session on AMS. Papers were presented by the Rochester-Toronto-General Ionex group, the Argonne National Laboratory, Oxford University, the Chalk River Nuclear Laboratories, AERE Harwell, and the University of Washington. The proceedings of these conferences are published in the journal RADIOCARBON, and Renee S Kra, whom this 40th Anniversary Special Issue of RADIOCARBON hon- ors, was its managing editor from 1968 to 1997. It was she, Minze Stuiver, and Austin Long who edited the proceedings of the 10th and many subsequent issues of the proceedings, all of which have contained papers on AMS. The 11th of these conferences was held in Seattle in 1982, the 12th in Trondheim in 1985, the 13th in Dubrovnik in 1988, the 14th in Tucson in 1991, the 15th in Glasgow in 1994, the 16th in Groningen in 1997, and the 17th will be held in Israel in 2000. Two books have been published on AMS recently. The first is a comprehensive technical account titled Accelerator Mass Spectrometry: Ultrasensitive Analysis for Global Science (Tuniz et al. 1998) and the second is a more popular account From Hiroshima to the Iceman: The Development and Applications of Accelerator Mass Spectrometry (Gove 1999). The first book is divided into three major parts— AMS equipment and techniques, principles of cosmogenic radionuclide production and distribution, and applications of AMS. It addresses in some detail virtually all the technical Some Comments on AMS 129 aspects of AMS. The second covers the historical development of AMS and of electrostatic accelerators, the instrumentation for AMS and a number of interesting applications.

Instrumentation for AMS Most AMS systems presently employ tandem electrostatic accelerators with cesium negative ion sputter sources. The samples or targets used in these sources are solids, e.g. in the case of 14C, a variety of techniques have been devised for converting small organic samples into graphite suitable for use in a sputter source. For radioisotope measurements sample sizes of 1–10 mg are employed, and generally, several samples can be mounted for insertion into the source without disturbing the vacuum system involved. The chemical forms generally used for each of the most common AMS radioisotopes are BeO, carbon (graphite), Al2O3, AgCl and AgI, respectively. The negative ions emerge from the source with energies of 25 keV or so. They are then passed through a magnetic analyzer. The addition of an electrostatic analyzer preceding the magnetic analyzer is used in some installa- tions. The ions are then further pre-accelerated to energies up to a few hundred keV depending on the size of the tandem and injected into the first half of the tandem focused on the stripper canal in the center of the high voltage terminal. The stripper canal is, typically, a differentially pumped tube into which argon gas is fed. The terminal of the tandem is held at a constant positive voltage ranging from 2 to 10 MV. Of the five most commonly measured radioisotopes only 36Cl requires the highest terminal voltage. In the terminal stripper, in the interactions between the energetic negative ions being accelerated and the stripper gas, not only is the extra electron that made the accelerated ions negative removed but additional electrons are stripped off, creating positive ions of charge 3+ or higher. The multiply charged positive ions then undergo a second acceleration from the terminal to ground. Positive ion analysis systems vary from one AMS laboratory to another. All of them, however, include both magnetic and electrostatic fields. A variety of detectors are employed following the magnetic and electric deflection analyzers at the high energy end of the AMS systems. The most common for the lightest radioisotopes such as 14C is a ∆E–E Si surface barrier solid state detector. For intermediate mass radioisotopes such as 36Cl a multi-anode gas ionization detector is employed. In such a detector both E and dE/dx are measured and it is possible to discriminate between isobars. In some cases, e.g. for 129I, a system for measuring the time of flight of the ions is employed to reduce the interference of tails of ions of nearby mass (127I in the case of 129I). AMS measurements consist of establishing the ratio of the counting rate of the radioisotope in question to the current of its stable isotope. Standards of known isotope ratio are run frequently for normalization, and blanks with no detectable radioisotope are employed as a measure of background. Corrections for other effects can be made by comparison with the standards. Various factors can affect the precision of an AMS measurement, the most important of which is counting statistics. The precision of most AMS measurements lies between 3% and 10%. 14C is an exception. Here a well-operated AMS facility can achieve a precision of 1% (±83 yr) and, if pressed and the sample warrants, 0.5% or 5‰ (±41 yr) or even better. There were 39 full-time or part-time AMS laboratories throughout the world in 1999. All but two employ tandem electrostatic accelerators. Of these 37, sixteen have terminal voltages between 2.5 and 3 MV, ten between 5 and 6 MV, five between 8 and 9.5 MV, and six between 12 and 20 MV. Except for AMS measurements of 36Cl, which require tandems with terminal voltages of 8 MV or higher (to resolve the stable isobar 36S) all other light and heavy radioisotopes and stable isotopes 130 H E Gove can be readily measured with tandems having terminal voltages as low as 2.5 MV. Some of the interesting measurements made recently by AMS follow.

The Atomic Bombing of Hiroshima The bomb that leveled Hiroshima was dropped on August 6, 1945. By the end of the year, 140,000 people in that city were dead. It was one of a kind and so had never been tested. It employed a gun barrel in which a “bullet” of the rare isotope of uranium, 235U, was fired against a 235U target. By the mid 1980s there were approximately 60,000 Japanese citizens who had been exposed to and survived the radiation from the Hiroshima bomb. These survivors as well as the ones from the bombing of Nagasaki are the best studied human beings in history in terms of the short- and long-term effects of nuclear radiation. For such studies to be meaningful, however, one must have a good estimate of the nature and degree of the nuclear radiation they received. In the case of the Hiroshima bomb no one knew with any accuracy at all what its neutron and gamma-ray yield might have been. Computer modeling was carried out in 1986 to predict these yields. The gamma-ray intensity emitted in the explosion could be measured as a function of distance from the explosion by a technique called thermoluminescence dosimetry. These measurements were made in roof tiles some omni- scient Japanese scientists had collected and stored, carefully cataloging them according to their distance from the bomb’s hypocenter and the extent to which they were shielded from the bomb by nearby buildings. The computer calculations were in general agreement with the measured gamma- ray intensities. These calculations, however, showed that neutrons played virtually no role in the radiation damage to humans. It was realized in the late 1980s by scientists at the Lawrence Livermore National Laboratory (LLNL) in California and at the Technical University in Munich, Germany that 36Cl produced by neutron capture in 35Cl was ideal for measuring the bomb neutrons. Chlorine is reasonably ubiquitous and can be found, at low concentrations, in building materials in Hiroshima that survived the nuclear holocaust. Because 36Cl has such a long half life (3 × 105 yr) whatever amount of it was produced in 1945 is still there to this day. A collaboration between scientists at Hiroshima University and the Technical University of Munich, using the Munich AMS facility, resulted in the measurement of the 36Cl to chlorine ratio (and thus the slow neutron fluence) in a granite gravestone 107 m from the hypocenter (slant range of 590 m) of the Hiroshima explosion (Kato et al. 1988). Although the measurement is interesting it gives no clues about the effect of such neutron doses on humans since everyone at that distance from the explosion died instantly from blast and heat. At the suggestion of a scientist at LLNL, five samples of building material collected from Hiroshima at slant ranges varying from 650 to 1700 m were measured at Rochester. When the results were translated into thermal neutron flux densities they showed that a person at the 650 m distance would have received a blast of neutrons in the split second of the explosion equal to the thermal neutron dose permitted for human beings to receive in a period of over 500 yr. Even at the largest distance of almost 2 km the neutron flux density was equivalent to a permitted thermal dose of 2 yr. The latter was almost a factor of 100 greater than the computer calculations had indicated. It was neutrons that had made the major contribution to the radiation damage suffered by the Hiroshima survivors, after all, and not gamma rays. This data was of the greatest significance and was published in the journal Health Physics (Straume et al. 1992). Some Comments on AMS 131

More recently, additional measurements of the 36Cl to stable chlorine ratio have been made at the Livermore AMS laboratory on Hiroshima samples (Straume 1993). The results agree with the original Rochester data and now present a reasonably complete picture of the slow neutron fluence emitted by the Hiroshima atom bomb over distances at which there were substantial numbers of survivors. What remains to be done is to evaluate the new information on the radiation these Hiroshima survivors actually received and to compare the doses received with the biological damage that resulted. It seems remarkable that it took almost 50 years for this to happen—and it would not have happened yet but for the development of AMS in 1977.

The Initial Peopling of the Americas One of the great contributions that carbon dating in general, and AMS carbon dating in particular, has made is establishing the dates of early human habitations. Such sites contain flint artifacts used for weapons and tools, animal and human bones in which, occasionally, are found imbedded flint weapon points, and camp fire sites containing samples of carbon from wood that was contemporary with the time the fires were used. Many times the samples of both bone and carbon are so small that only AMS can provide a date. Many such human habitation sites have been found in North and South America. In North America the oldest sites dating back almost 13,000 years are found in Alaska. Other sites are found in Mon- tana, Wyoming, South Dakota, Colorado, Oklahoma, New Mexico and Arizona and they have later dates of around 12,000 yr. This strongly suggests that early humans crossed from Siberia to Alaska via the Bering Strait, which at the time may have had a land bridge, and moved south via a corridor that existed between the Laurentide and Cordilleran ice sheets well before the last ice age ended some 7000 years ago. Two fairly recent discoveries in this connection are particularly noteworthy. The first is the discovery of a site of human habitation in Monte Verde, Chile. Monte Verde is some 575 miles south of Santiago, Chile and, as the crow flies, some 20,000 miles from the Siberian-Alaskan land bridge. The site has been carbon dated to be some 13,500 years old or perhaps even older (Dillehay 1989, 1997). It is at the southern limit of continental Chile approximately 30 miles east of the shores of the Pacific Ocean. Further south complex waterway channels and islands stretch some 600 miles to Tierra del Fuego. It seems a most unlikely site for the oldest habitations of people in the Americas, raising as it does the question of how its inhabitants could possible have gotten there. The second discovery, in the summer of 1996 on the banks of the Columbia River near Kennewick, Washington some 230 miles from the river’s mouth on the Pacific Ocean, was what eventually became a 90–95% complete skeleton of a man (Gibbons 1996). The land on which the skeleton was discovered is owned by the Army Corps of Engineers. The skeleton had two fascinating aspects: a projectile point was imbedded in his pelvis and, morphologically, his skull did not look Native American—his features were caucasoid. Collagen, extracted from a sliver of bone was measured in the AMS laboratory at LLNL (R E Taylor, personal communication 1997). It was some 9000 years old. It is believed that the bone sliver is so well preserved that it may be possible to extract and ana- lyze DNA. Such a test could answer the question of to what group of people Kennewick belonged. Was he Caucasian or an Asian ancestor of American Indians? The Confederated Tribes of the Uma- tilla Indian Reservation, the local group of Native Americans, made the astonishing claim that he was an ancestor and should undergo ceremonial internment. All further tests are suspended pending a decision by the appropriate court. 132 H E Gove

The Neolithic Iceman In September 1991, two German hikers discovered a frozen corpse in the Similaun glacier of the Alps on the border between Austria and Italy 10,500 feet above sea level. Dubbed the Iceman, the corpse resembled the emaciated and shrunken bodies found in concentration camps at the end of World War II. When it was finally freed from its icy tomb the body, still frozen solid, and a number of artifacts found with it were transported to the Research Institute for Alpine Studies in Innsbruck (Spindler 1993). The artifacts included a 10-cm copper ax head with its handle and bindings intact, a 1.82-m-long incomplete wooden bow made of yew but broken during the body’s recovery, eleven 85 cm-long arrows—two of them tipped with double-faced flint points, the world’s oldest quiver made from deer skin, an ash-handled flint dagger with its grass sheaf, a bone needle, and a 25-cm stick tipped with an antler. This remarkable find constituted one of the major anthropological discoveries of the century. As such it was of paramount importance to establish its age. Carbon dating was clearly the way to do so. However, before the development of AMS, an unacceptably large sample of his flesh and bones would have been required for radioactive decay counting by the Libby method exclusively employed up to 1977. However, AMS was by then and had been for many years a mature science and could be safely employed to date the Iceman with minimal destruction of precious material. A number of European carbon dating laboratories were involved, including Gif-sur-Yvette in France, Oxford in England, Uppsala in Sweden, and Zurich in Switzerland. Excellent agreement was obtained by the four laboratories. Because of wiggles in the 14C dendrochronology calibration curve, the age of the Iceman could not be pinned down with the accuracy inherent in AMS. The best that can be said is that he died between 3110 and 3370 BC, or some 5240 years ago. The botanical material was also carbon dated and the results have been published (Prinoth-Fornwagner and Niklaus 1994).

Other Recent AMS Results The above are but three of many areas of recent research that have benefited from the power of AMS. Others include the measurement of the leakage of nuclear waste from US Department of Energy nuclear fuel reprocessing plants at the Savannah River Site in South Carolina, the Hanford Reservation in Washington, the Idaho National Engineering Laboratory in Idaho, and West Valley in New York (see references in Gove 1999, chapter 9). Measurements of 36Cl and 129I by AMS in the soil and water in the vicinity of these plants shows that the some 11 million cubic feet of high-level nuclear waste distributed between them is leaking into the environment. Other measurements by AMS include the age of the Turin Shroud—the reputed burial cloth of Christ (640 years old) (Gove 1996), the Dead Sea scrolls (some 2000 years old), the Elephant Bird egg found in Australia in 1993 (1970 years old), and many other 14C dating results. Another important application of AMS lies in the field of biomedicine. This application was first suggested by two scientists at the University of Rochester’s Medical School in 1978 (Keilson and Waterhouse 1978). However, it took some 10 years before actual biomedical research employing AMS began. The pioneering work in this field was carried out at the Center for AMS at LLNL in California and the results of this and other AMS groups have been presented at AMS conferences beginning with the fifth in Paris in 1990 as well as in many other medical related journals. Some Comments on AMS 133

Thoughts on Future Developments in AMS Two possible new developments will be described. The first involves 14C background measurements of interest in connection with the very large volume scintillation detectors planned, being built or in operation as neutrino detectors. The second involves the use of neutral ion injection for AMS (NI AMS). It was mentioned above that one of the reasons for developing AMS for the detection of 14C was the hope that it could date organic samples that “died” much longer ago than the 60,000-year or so age limit of the Libby decay counting method. Since the latter limit is probably due to cosmic ray background in the counters used to detect the β-decay of 14C and AMS equipment is not sensitive to such a background, AMS measurements should date back much further in time, perhaps to as far back as 100,000 years. Carbon of contemporary organic origin has a 14C/C ratio of 1.2 × 10−12. The ratio for 60,000-yr-old carbon is 8.5 × 10−16. That for 100,000-yr-old carbon is 6.7 × 10−18. An analysis of the background for 14C AMS has been made at the University of Toronto’s AMS laboratory (Beukens 1992). The 14C/C ratio in a blank aluminum sample was 2.5 × 10−17. The ratio in several samples, one from CO2 from natural gas, one from marble supplied by the IAEA, and one from anthracite, were statistically similar and yielded a value of about 10−15. Some decay counting laboratories achieve values this low or lower. These ratios, however, are considerably higher than one would expect in the samples tested. Some 14C must be present in natural gas and petroleum due to the very low levels of muons and natural radioactivity in many locations deep in the earth. Estimates of the ratio 14C/C are near 10−20 from such sources and will vary greatly due to the presence or absence of nearby uranium or thorium deposits. The lowest ratio measured by AMS was 10−18 in 1991 at the University of Toronto’s IsoTrace Laboratory (R P Beukens and R S Raghavan, personal communication 1991). The sample was carbon monoxide obtained from natural gas enriched in 14C by a factor of 104. The material was supplied to IsoTrace for analysis in connection with the Borexino project at the Gran Sasso underground laboratory. Later, reported in 1998, the 4 Mg test liquid scintillator there recorded a level of 2.0 × 10−18 for 14C/C by decay counting (Alimonti et al. 1998). This is the lowest 14C abundance ever measured in a non enriched sample. It is believed that the above AMS ratio of 10−18 in the −20 enriched CO2 sample can be reduced to 10 or better. This is highly desirable for three intercon- nected reasons: 1) The choice of carbon for liquid scintillators, for neutrino measurements, will be facilitated, 2) the 14C background and contamination in AMS dating work will be better understood, and 3) the variability of the deep underground production of 14C can be studied. The second possible future development involves neutral injection for AMS (NI AMS). This possibility has been suggested previously (Litherland and Kilius 1997; Litherland et al. 1999). When the electron affinity of a nuclide is low or negative, or when the wanted nuclide is imbedded in an insulator, the initial production of singly charged positive ions directly in an ion source or by bombardment by negative ions is highly advantageous. Neutral injection involves converting effi- ciently these positive ions to neutrals by resonant electron transfer. The cross sections for the formation of neutral atoms from singly charged ions are very large when the projectile ions collide with stable atoms of the same species or when the binding energies of the electrons of the projectile ion are close to those of the ground or an excited state of the bombarded stable ions of a different species. The resulting neutral ions are then injected into a tandem electrostatic accelerator. Singly charged positive ions are readily produced for most atoms and molecules unlike the situation for negative ions. 134 H E Gove

Neutral injection of atoms and molecules of the same mass to the terminal of a tandem followed by a charge change to 1+ in the terminal and then followed by a charge change to 3+ at ground (needed for molecular breakup of the molecular isobar(s) has an additional advantage over standard AMS (negative ions injected into the tandem followed by charge change to 3+ in the terminal). It affords a better mass energy product separation between the wanted atom and the molecular fragments from the molecular isobar. When the wanted atoms are imbedded in an insulator, for example metals in a silicate rock matrix, the use of singly charged positive Cs ions to bombard the insulator to produce the wanted singly charged negative ions produces a buildup of charge. This causes voltage instabilities that invalidate the measurements. When negative ions are employed to bombard the insulator to produce the wanted singly charged positive ions there is little or no buildup of charge. This is probably due to the mobility of the electrons created by the negative ion bombardment and their being driven off by the electric field of the negative ions. The vast majority of elements in the periodic table have either low or negative electron affinities (the binding energy for attaching an electron to a neutral atom). Most of these can readily be produced as singly charged positive ions. They include the lanthanides (Z = 57–71), the transition elements (Z = 21–30, Z = 39–48, and Z = 72–80), the noble gases (Z = 10, 18, 36, 54 and 86) and the actinides (Z greater than or equal to 89). There are other special cases such as cesium isotopes that are more easily formed as singly charged positive ions than singly charged negative ions and many of the plat- inum group elements (PGE)—a sub set of the transition elements—whose detection at very low levels in situ in insulating rock matrices is of importance. NI AMS, if implemented, might well constitute a new revolution in accelerator mass spectrometry.

ACKNOWLEDGMENTS I would like to acknowledge invaluable discussions with A E Litherland, R P Beukens and other colleagues at the University of Toronto’s IsoTrace Laboratory, and K H Purser of Southern Cross Cor- poration. I would also like to pay tribute to Renee Kra for her sterling editorial contributions to the field of radioisotope measurements, particularly radiocarbon, and to her charm and her indomitable spirit.

REFERENCES Alimonti G et al. 1998. Measurement of the 14C abun- trometry for measurements of long-lived radioiso- dance in a low-background liquid scintillator. Physics topes. Science 236:543–50. Letters B422:349–58. Fifield LK, Fink D, Sie SH, Tuniz C, editors. 1993. Pro- Beukens RP. 1992. Radiocarbon accelerator mass spec- ceedings of the 6th International Conference on Ac- trometry: background, precision and accuracy. In: celerator Mass Spectrometry. Nuclear Instruments Taylor RE, Long A, Kra RS, editors. Radiocarbon af- and Methods in Physics Research B92:1–524. ter four decades: an interdisciplinary perspective. Gibbons A. 1996. The peopling of the Americas. Science New York: Springer-Verlag. p 23–239. 274:31–3. Dillehay, TD, editor. 1989. Monte Verde, a late pleis- Gove HE, editor. 1978. Proceedings of the first confer- tocene settlement in Chile. Vol I. The archaeological ence on radiocarbon dating with accelerators. Roch- context. Washington DC: Smithsonian Institution ester, NY: University of Rochester. Press. Gove HE. 1985. Accelerator-based ultrasensitive mass- Dillehay TD, editor. 1997. Monte Verde, a late pleis- spectrometry. In: Bromley DA, editor. Treatise on tocene settlement in Chile. Vol II. Paleoenvironmental heavy ion science. Vol ume 7 . N ew Yo rk : P lenu m and site context. Washington DC: Smithsonian Insti- Press. p 431–563. tution Press. Gove HE. 1992. The history of AMS, its advantages over Elmore D, Phillips FM. 1987. Accelerator mass spec- decay counting: applications and prospects. In: Taylor Some Comments on AMS 135

RE, Long A, Kra RS, editors. Radiocarbon after four Research B123:18–21. decades: an interdisciplinary perspective. New York: Litherland AE, Purser KH, Gove HE. 1999. Neutral in- Springer-Verlag. p 214–29. jection for radioactive beams and accelerator mass Gove HE. 1996. Relic, icon or hoax? carbon dating the spectrometry. In: Shepart KW, editor. CP473, Heavy Turin Shroud. Bristol and Philadelphia: Institute of Ion Accelerator Technology: 8th International Confer- Physics Publishing. 336 p. ence. The American Institute of Physics. Gove HE. 1999. From Hiroshima to the Iceman: the de- Prinoth-Fornwagner R, Niklaus Th R. 1993. The man in velopment and applications of accelerator mass spec- the ice: results from radiocarbon dating. Nuclear In- trometry. Bristol and Philadelphia: Institute of Physics struments and Methods in Physics Research B92:282– Publishing. 226 p. 90. Gove HE, Litherland AE, Elmore D, editors. 1987. Pro- Purser KH, Liebert RB, Litherland AE, Beukens RP, ceeding of the 4th International Symposium on Accel- Gove HE, Bennett CL, Clover MR, Sondheim WE. erator Mass Spectrometry. Nuclear Instruments and 1977. An attempt to detect stable N− ions from a sput- Methods in Physics Research B29:1–455. ter ion source and some implications of the results for Jull AJT, Beck JW, Burr GS, editors. 1996. Proceedings the design of tandems for ultra-sensitive carbon anal- of the 7th International Conference on Accelerator ysis. Revue de Physique Appliquee 12:1487–92. Mass Spectrometry. Nuclear Instruments and Meth- Spindler K. 1993. The Iceman’s last weeks. Nuclear In- ods in Physics Research B123:1–612. struments and Methods in Physics Research B92:274– Kato K, Habara M, Aoyama T, Yoshizawa Y, Biebel U, 81. Haberstock G, Heinzl J, Korschinek G, Morinaga H, Straume T. 1993. Neutron discrepancies in the DS86 do- Nolte E. 1988. Measurement of the neutron fluence simetry system have implications for risk estimates. from the Hiroshima atomic bomb. Journal of Radia- RERF Update 4:3–4 (Hiroshima: Radiation Effects tion Research 29:261–6. Research Foundation). Keilson J, Waterhouse C. 1978. Possible impact of the Straume T, Egbert SD, Woolston WA, Finkel RC, Kubik new spectrometric techniques on 14C tracer kinetic PW, Gove HE, Sharma P, Hoshi, M. 1992. Neutron studies in medicine. In: Gove HE, editor. Proceedings discrepancies in the DS86 Hiroshima dosimetry sys- of the 1st Conference on Radiocarbon Dating with Ac- tem. Health Physics 63:421–6. celerators. University of Rochester. p 314–9. Thompson JJ. 1912. XIX Further experiments on posi- Kutschera W, editor. 1981. Proceedings of the Sympo- tive rays. Philosophical Magazine 24:209–53. sium on Accelerator Mass Spectrometry. ANL/PHY Tuniz C, Bird JR, Fink D, Herzog GF. 1998. Accelerator 81-1. Springfield: National Technical Information mass spectrometry: ultrasensitive analysis for global Service, US Department of Commerce. science. Boca Raton, Florida: CRC Press. 371 p. Litherland AE. 1980. Ultrasensitive mass spectrometry Woelfli W, Polach HA, Anderson HH, editors. 1984. Pro- with accelerators. Annual Reviews of Nuclear and ceedings of the 3rd International Symposium on Ac- Particle Science 30:437–73. celerator Mass Spectrometry. Nuclear Instruments Litherland AE. 1984. Accelerator mass spectrometry. and Methods in Physics Research B52:91–448. Nuclear Instruments and Methods in Physics Re- Yiou F, Raisbeck GM, editors. 1990. Proceedings of the search B5:100–8. 5th International Conference on Accelerator Mass Litherland AE, Kilius LR. 1997. Neutral injection for Spectrometry. Nuclear Instruments and Methods in AMS. Nuclear Instruments and Methods in Physics Physics Research B52:211–630.

RADIOCARBON CALIBRATION AND APPLICATION TO GEOPHYSICS, SOLAR PHYSICS, AND ASTROPHYSICS

Paul E Damon • Alexei N Peristykh Department of Geosciences, Gould-Simpson 208, the University of Arizona, Tucson, Arizona 85721 USA. Email: [email protected]; [email protected].

ABSTRACT. This paper includes a brief history of the calibration of the radiocarbon time scale from the first recognition of the necessity of calibration in 1962 to INTCAL98. Thirty-six years of effort by dendrochronologists and the 14C community have pushed the tree-ring calibration back to 11,854 yr BP. All of this part of the calibration has been done by high-precision beta counting. Uranium-thorium (U-Th) dating of coral samples coupled with accelerator mass spectrometry (AMS) measurement of 14C has extended a fairly detailed calibration back beyond the Bølling warm episode to 15,000 BP. Earlier than 15,000 BP, piecewise linear approximation extends INTCAL98 calibration to 24,200 BP. Blending 1-, 2-, 3-, 10-, and 20-yr tree-ring samples containing regional and data offsets into a decadal time scale does not make an ideal error and bias free ∆14C record. Nevertheless, spectral analysis reveals some statistically significant fundamental frequencies as well as interesting “beat” frequencies and the second harmonic of the around 208-yr cycle that is considered to be solar in origin. Although, some very prominent peaks such as the 88-yr (Gleissberg) are clearly solar in origin, some of the lower frequencies such as of the 512-yr period may have an origin in thermohaline circulation. Thus, INTCAL98 provides useful data for geophysical and solar physics research. Lastly, single year ∆14C analysis would be useful for revealing invaluable information for solar physics, astrophysics and geophysics not accessible by decadal data. We provide several examples.

History of the Calibration of the Radiocarbon Time Scale Hessel de Vries (1958, 1959) was the first to provide evidence for preindustrial secular variation of atmospheric 14C. His data showed an increase in 14C during what we now refer to as the Maunder Minimum, but an offset was apparent in the comparison of measurement on tree rings from Colo- rado and Germany. When corrected for isotope fractionation the offset was removed and the combined data demonstrated an increase of about 15‰ during the Maunder Minimum (Lerman et al. 1970). The “De Vries effect” (Damon and Long 1962) was soon confirmed (Broecker and Olson 1959) and it was demonstrated that the De Vries effect wiggles are superimposed on a higher amplitude longer trend (Ralph and Stuckenrath 1960; Suess 1961; Damon et al. 1963). At the same time precise and accurate determinations of the 14C half-life were being made (Mann et al. 1961; Watt et al. 1961; Olsson et al. 1962). A new half-life was confirmed at the 5th Radiocarbon Conference held at the University of Cambridge in 1962 (Godwin 1962). The new half-life of 5730 ± 40 yr was significantly different from the Libby half-life of 5570 yr. The question raised at that conference was how to report dates in the future. The first author (PED) attended that conference and he pointed out the problem in agreeing on the value of the curie, which was tied to the disintegration rate of a gram of radium. At each conference someone had determined a new disintegration rate. After numerous conferences, it was agreed to detach the curie from the activity of radium and arbitrarily fix it at 3.7 × 1010 dps. The senior author could visualize the confusion of reporting 14C dates with a new half-life and setting precedent for future changes when it was clear that, even with the new half-life, 14C years would not be exactly equal to calendar years! It seemed best to retain the Libby half-life and make the best correction to the calendar years available at the time of publication. Fortunately, there was general agreement on this procedure and the announcement by Godwin (1962) made it official. The importance of calibration became very evident. By the time of the 12th Nobel Symposium, which was held 11–15 August 1969 (Olsson 1970) a 7484-yr bristlecone pine chronology had been developed by Ferguson and the 14C trend curve had been extended by the Arizona and Pennsylvania laboratories back to the beginning of the 5th century

137 138 P E Damon, A N Peristykh

BC showing that 14C dates with the Libby half-life were about 800 yr younger than calendar years. The new half-life would correct for only 180 yr of the discrepancy. In addition, the three major causes of atmospheric 14C fluctuation had been identified. These were: 1) changes in the intensity of the geomagnetic dipole moment, 2) heliomagnetic modulation, i.e., modulation by the magnetic field imbedded in the solar wind, and 3) changes in the carbon cycle. A prescient article by Lingenfelter and Ramaty (1970) also included the production of 14C by solar flares and supernova. Two plates were included showing the available and reasonably reliable calibration data. Plate I included only the La Jolla data back to around 2200 BC showing what would later be referred to as the Suess wiggles (De Vries effect) with a calibration curve drawn by eye with his famous “Cosmic Schwung”. The second was a plot of all the other data. The Arizona, Groningen, and Pennsylvania laboratories data were published in the symposium volume. All of these were normalized to the international standard NBSI. In addition, the data from Cambridge, Copenhagen, and Heidelberg (Willis et al. 1960), for tree rings from AD 659 to 1859, were included. Incidentally, their paper was the first to suggest a 200-yr cycle. The publication of this data gave rise to many smoothed and wig- gly calibration curves. Actually, these calibration curves were adequate enough to justify Renfrew’s (1973) declaration of a second revolution in 14C dating. The first occurred when 14C dating proved the greater antiquity of the European and Near East Neolithic period, setting its beginning some 3000 yr earlier than previously hypothesized within the traditional “short chronology”. 14C dating was instrumental in introducing a long chronology. The second revolution took place when the calibration of dates obtained on archaeological materials indicated that the European cultures did not postdate the Middle Eastern cultures as the diffusionists predicted. The second revolution was harder to accept than the first. A third revolution occurred with the advent of the determination of 14C by accelerator mass spectrometry (AMS) in 1977 (Nelson et al. 1977; Bennett et al. 1977). The new technique would make possible the 14C dating of sub-milligram samples of 14C and more rapid analysis. This allowed for greater selectivity in sampling and the dating of minute samples that cannot be dated by beta counting. Nevertheless, calibration by beta counting would continue because of the high precision obtainable with sufficiently large samples. During the subsequent decade following the 12th Nobel symposium a dozen or so calibration schemes were published. This resulted in considerable confusion in the literature. Consequently, the National Science Foundation (NSF) funded a workshop held in Tucson during the winter of 1979. The purpose was to arrive at a single 14C calibration scheme involving graphs and tables. Twenty- one people attended including representatives of laboratories funded by the NSF, dendrochronologists and statisticians. Although the De Vries effect had been amply verified for the last millennium, no single De Vries fluctuation or “Suess wiggle” had been confirmed by two or more laboratories prior to the last millennium. Consequently, a second goal was to verify the Suess wiggles prior to the last millennium. This was particularly important because of the significance of the De Vries effect for solar physics (Damon et al. 1980). There was clear evidence of the continuation of the “Suess wiggles” (De Vries effect) back to 5000 BC. All but a few of the larger “Suess wiggles” were confirmed. However, although four of the laboratories calibrated to the NBS oxalic acid international standard agreed within ± 0.3‰ (± 2.5 yr) of the average of the four (Arizona, Groningen, Pennsylvania, and Yale), one laboratory (La Jolla) that calibrated to a laboratory standard diverged from the average by 6‰ (49 yr too old). Consequently, a correction was required (Klein et al. 1980). Details of the resultant calibration are given in Klein et al. (1982). The calibration was based on 1154 sampler of dendrochronologically dated wood from the five laboratories previously mentioned. The Application to Geophysics, Solar Physics, Astrophysics 139 calibration was presented both as graphs and as a table with 14C dates every 10 yr and 95% confidence ranges for 6 different measurement standard deviations. This calibration scheme served for only 6 yr. Beginning with Pearson et al. (1977) and De Jong et al. (1979), a shift had begun to higher precision calibration about ±2‰. The first calibration issue of Radiocarbon (Stuiver and Kra 1986) included high precision data from six laboratories (Arizona, Belfast, Groningen, Heidelberg, Pretoria, and Seattle) and a computerized 14C age calibration (Stuiver and Reimer 1986). Measurements on dendrochronologically dated German oak and US bristlecone pine extended the calibration back to 7200 BC (9150 BP). A more detailed account of the history of calibration of 14C dates by dendrochronology through the first calibration issue in 1986 may be found in Damon (1987). The second calibration issue of Radiocarbon (Long et al. 1993) included an attempt to push the Ger- man oak and pine chronology back to 11,400 BP (Becker 1993), corrections to the Belfast 14C data (Pearson and Qua 1993) and to the Seattle data (Stuiver and Becker 1993), plus comparison of AMS measurement of 14C and thermal ionization mass spectrometry (TIMS) measurements of 230Th and 234U on coral to provide a first-order calibration of the 14C time scale back to 30,000 yr (Bard et al. 1993). A controversial shift of 20 yr older in the Belfast data resulted from the removal of two standards that were suspect. The correction in the Seattle data for radon was time dependent resulting in an age increase of only 10 14C yr for samples a few hundred years old, an increase of 30 yr for 4500- yr-old samples. Fortunately, careful records were maintained which permitted accurate corrections to be made. The first-order calibration of the 14C time scale by U/Th-14C measurements on coral was a new very important development. 14C dates were shown to be too young by 4600 ± 600 (2 σ) yr at 30,000 yr or a depletion of about 550‰. The authors point out that this confirms paleomagnetic evidence for a very low dipole field intensity at that time. However, they estimate that 10–20% of the discrepancy between the two chronometers may involve changes in the carbon cycle. The Calibration 1993 issue of Radiocarbon was followed 5 yr later by the INTCAL98 calibration issue of Radiocarbon (Stuiver and Van der Plicht 1998). The most significant development in the dendrochronologic calibration of the 14C time scale was in the Hohenheim oak chronology (Spurk et al. 1998) that was found to be in significant error prior to 5242 BC (pre-7192 BP). Correction of these errors resulted in a shift toward older ages of 41-yr pre-5242 BC (pre-7192 BP) and a shift of 95 year (41 + 54) pre-7792 (pre-9742 BP). Thus, after much labor over 40 yr, an improved dendrochronologically dated time scale was being forged. However, as has been said, “the devil is in the details”. It has become increasingly apparent that a global 14C time scale accurate to better than ±50 yr is not possible due to hemispheric and regional offsets. A short review of the literature on this subject was given by Stuiver et al. (1998:1045–6). Three processes have been reported to produce a measurable regional change in the 14C/C ratio of the atmosphere. There is a latitude effect (Braziunas et al. 1995), costal upwelling (Damon et al. 1989, 1999; Damon 1995) and thawing of frozen earth (Damon et al. 1996). The latitude effect is negligible from the Equator to the Arctic Circle but increases from the equator to around 65°S where it has been calculated to be −6‰ (50 yr too old) (Braziunas et al. 1995). However, in regions of coastal upwelling, such as western South Africa, depletion can be greater than the predicted latitude effect. More or less strong westerly winds result in an Ekman spiral (Ken- nett 1982). The spiral upon reaching the coast of Southern Africa results in the strong Benguela cur- 14 rent that produces divergence and upwelling of deepwater rich in nutrients and C-depleted CO2. If the upwelling is sufficiently strong and continuous it can reduce the 14C/C ratio in the prevailing air mass sufficiently enough for the depletion to be measured in tree rings. So far, we have measured 44 (±3‰) single-year tree rings of Widdringtonia cedarbergensis from Die Bose, Cape, South Africa 140 P E Damon, A N Peristykh

(Damon et al. 1999). These measurements are compared with the same years from trees (4 Douglas- firs and 1 Noble fir) growing in the Pacific Northwest (Stuiver et al. 1998). The results from 1794 to 1829 are in close agreement with the predicted latitude effect of 2.9 ± 0.5‰. However, following 1829 there is a rapid decrease to very low values with an average depletion for the 8 yr from 1830 to 1837 of 9.4 ± 0.8‰ (78 ± 7 yr). The latitude effect alone would only account for about a 24-yr decrease in age. However, the frame of reference is to the Pacific Northwest where coastal upwelling also takes place (Kennett 1982). The climatic implications are interesting but detailed discussion is beyond the scope of this paper. Data published by Vogel et al. (1993) in Calibration 1993 show that depletion in excess of the latitude effect continued until 1890 after which the situation is complicated by the Suess effect. Their data from 1853–1890 show a depletion of 5.5 ± 0.6‰ compared to the predicted latitude effect of 3.1 ± 0.5‰ leaving an excess due to upwelling of 2.4 ± 0.8‰. Thus, upwelling significant enough to deplete the prevailing air masses in 14C continued from 1830 to 1890 or a duration of 60 yr. As a consequence, westerly winds must have driven the Ekman spiral and Benguela current more intensely. Another very significant aspect of INTCAL98 is further contributions to the extension of the 14C time scale by U/Th and 14C measurements on coral. There is good agreement between the tree-ring and coral data with the corrected dendrochronology. Bard et al. (1998) point out that ages beyond U/ Th calibrated 10,000 BP including the two points at U/Th age 41,100 ± 50 BP and 30,230 ± 160 BP can be approximated by a simple linear equation: [cal BP] = 1.168 × [14C-age BP]. A second-order polynomial provides a better fit. Significant fluctuations from a linear trend can be seen in the coral data of Bard et al. (1998) and Burr et al. (1998). Figure 5 of Burr et al. (1998) shows unacceptable divergence between the tree-ring data and varve data from Sweden (Wohfarth 1996) and Lake Go ci¹¿ in Poland (Goslar et al. 1995) but much closer agreement with Cariaco marine varve data (Hughen et al. 1998). Because of this, only the Cariaco varve data were included in INTCAL98. The various corrections and adjustments necessary to arrive at the INTCAL data base are described in detail by Stuiver et al. (1998). The resultant data merged in decadal intervals and presented in per mil differences are shown in Figure 1 below. Terrestrial varve chronologies are planned to be discussed in a future calibration issue of Radiocarbon.

Calibration Curve and its Fine Structure The ∆14C versus calendar age in Figure 1 can be divided into three parts. Prior to 9846 BC, the curve is dependent on the coral data and varve data from the Cariaco Basin. Prior to 5242 BC there are significant changes in the Hohenheim dendrochronology, and after 5242 BC, there is no change in the dendrochronology. Work on the ∆14C of coral began during the last decade of the 20th century, whereas data using dendrochronologically dated tree rings has been obtained over about four decades. The resultant trend is generally considered to be from changes in the intensity of the Earth’s dipole moment and the decay of a large inventory of 14C accumulated as a result of a very low average dipole moment during most of the preceding late ice age. However, Stuiver et al. (1998) point out that the pronounced minimum of the coral ∆14C and pronounced maximum of δ18O occurs during the Bølling warm episode. This demonstrates that climate induced changes in the carbon cycle are an important factor during the Ice Age. The Suess wiggles around the trend have been referred to as the fine structure (Damon et al. 1989). The inset in Figure 1 shows the last millennium of data unfiltered. Observation of sunspots have demonstrated the absence or dearth of sunspots during the Maunder Minimum (Eddy 1976). The absence of sunspots is associated with a much lower solar wind intensity and an increase in the cosmic-ray intensity and consequent neutron flux that produces 14C by the reaction 14N(n,p)14C. Thus Application to Geophysics, Solar Physics, Astrophysics 141 the magnetic field imbedded in the solar wind has an effect similar to the Earth’s dipole field intensity. Both deflect cosmic rays from reaching the Earth and so there is both geomagnetic and heliomagnetic modulation of the neutron flux and consequent production of 14C and other cosmogenic isotopes. The relationship with 14C production is the inverse. Low solar activity or dipole field intensity is associated with an increase in 14C production. Consequently, the Maunder Minimum of solar activity results in high ∆14C (De Vries effect) during the Maunder Minimum. Other 14C maxima in the fine structure have also been associated with solar activity; these are named after astronomers (Oort, Wolf, Spörer, Maunder, and Dalton). The Oort and Wolf minima are separated by the Medi- eval Solar Maximum, which is contemporaneous with Lamb’s Late Medieval Warm Epoch (Lamb 1965). On the other hand, the Maunder Minimum has been associated with cooler climates by various authors. Consequently, there is the possibility of an irradiance component associated with the 14C Maxima and Minima (Eddy 1977).

Figure 1 ∆14C data back to 16,000 BP. The curve is based on INTCAL98 tree-ring data back to 11,854 BP. Prior to that the curve is based on marine data (coral and varves). The inset shows the unfiltered ∆14C data for the last millennium. The ∆14C maxima (solar activity minima) are named after unfiltered ∆14C data for the 1st millennium. The ∆14C maxima (solar activity minima) are named after astronomers. The Medieval Solar Maximum occurs between the Oort and Wolf Minima of solar activity.

Spectral Analysis of ∆14C Variations in the INTCAL98 Data The ∆14C obtained from coral is not yet at a stage that warrants spectral analysis. The dendrochronologically dated ∆14C data extend back to 11,854 BP. However, there are still a few gaps in the record prior to 7195 BC. Consequently, we confine spectral analysis to the data from 7195 BC to AD 1895. 142 P E Damon, A N Peristykh

Before analyzing the ∆14C data for significant periodicities in the fine structure, some pre-processing is required. It is necessary to remove the long-term trend which is thought to be due to the result of change in the intensity of the geomagnetic dipole moment. To calculate the long-term trend, low- pass filtering is required. For the purpose of low-pass filtering we chose a smoothing spline algo- rithm (Cook and Peters 1981). This method unlike the cubic interpolating spline, approximates the observational data (yi, i = 1,...,n) with rescaling weights δyi through minimizing the following func- tional (Reinsch 1967):

xn ∫ ["()]fxdx2 x1 (1) under the constraint

2 n  fx()− y  ii ≤ S, ∑ δ  yi  i=1 (2) where S is some given parameter specifying scaling. An advantage of this smoothing spline as a digital filter is monotony of its magnitude frequency response, which means it does not have ripples and, therefore, it produces no artificial peaks in Fou- rier spectrum. Moreover, its magnitude frequency response function can be expressed by a simple formula with only one given parameter which, being changed, defines a family of similar curves intersecting only at zero frequency. Hence, it is uniquely defined by specifying its any single point (except, of course, zero frequency). Therefore, one totally determines all properties of the magnitude spectrum of a smoothing-spline filter by specifying its gain (G, %) at some period (T, yr) of interest. Another great advantage of this technique is a zero phase shift (phase frequency response is equal everywhere to zero), which means it does not distort the shape of the signal. The smoothing spline to get long-term trend is determined by parameters T = 8000 yr, G = 99%. Figure 2 includes the detrended ∆14C data distinguishing the low pass components by smoothing spline (T = 1000 yr, G = 99%) from higher frequency components.

Figure 2 Detrended tree-ring ∆14C data (thin line) and further low-pass filtered by a smoothing spline of T = 1000 yr, G = 99% (thick line). Application to Geophysics, Solar Physics, Astrophysics 143

We used a Welch modified periodogram (Giordano and Hsu 1985) spectral estimator scheme, which uses averaging periodograms of weighted and overlapped data segments of the total record. The purpose of overlapping segments is to increase the number of segments that are averaged for a given data record length (N) and therefore, to decrease the variance of PSD estimate as a statistical variable. To equalize the treatment of most of the data samples, we used symmetric 50% overlap (for L = [N/K] – long segments that gives us 2 ⋅ K–1 segments). Chosen data windows were applied to the lin- early detrended data in each segment prior to the computation of the segment periodogram. The purpose of tapered windowing is to reduce the effect of side lobes and to decrease the estimation bias, at the price of a slight (<2) decrease in resolution. We used a Tukey-Hamming window:

 1 [(11++−⋅aa ) ( ) cos( 2π xLxL / )], ≤ / 2 wx()=  2   02,/xL>  (3) where a = 0.08 is data weight at the edges of the tapered window. The Tukey-Hamming window was preferred to the Hanning window (a = 0) because of its somewhat lower ratio of largest side lobe to main lobe amplitudes (–43 dB compared to –32 dB), whereas it has the same main lobe width 8π/L rad (Cadzow 1987). For the purpose of spectral peaks comparison, we plot spectra here as the square root of spectral power density. Unless otherwise specified, the method used will be the above described Welch’s weighted overlapped segment averaging (WOSA) spectral estimator. It is well established that changes in the “great ocean conveyor belt” (Broecker 1991) resulted in climate instability during the Last Ice Age and consequent changes in the carbon cycle. In contrast, Holocene climate has been relatively stable on a scale of the North Atlantic hydrologic cycle (e.g., see Figure 1 in Dansgaard et al. 1993). However, during the transition period following the Younger Dryas (ca. 9700 BC), the spline data in Figure 2 follows rather large fluctuations of the fine structure. This might suggest significant changes in the carbon cycle modulating 14C. However, Figure 14 in Stuiver et al. (1998) shows no reflection in the GISP2 δ18O record for the large peak at 9050 BC (see Figure 2 in this paper). Consequently, we will assume that during the Holocene the detrended ∆14C data is not dominated by large-scale climate induced changes in the carbon cycle. We will show that changes in solar activity are important as implied by the naming of the ∆14C maxima– solar activity minima after astronomers in the inset within Figure 1. This does not preclude a synchronous enhancement of the Maunder-type episodes by a Sun-climate component affecting the transfer of carbon from the mixed layer to deep sea. Stuiver and Braziunas (1993) refer to this as a Sun-ocean contribution to atmospheric ∆14C. There is a ∆14C climate correlation. Damon and Jir- ikowic (1992) and Stuiver et al. (1997) have shown that small changes in δ18O in Greenland ice cores correlate with the ∆14C events in Figure 1 inset. The INTCAL98 calibration curve in Figure 1 is not composed from consecutive decadal samples with the center of each decade spaced by exactly 10 yr. Even in decadal data from the same laboratory there are occasional offsets and samples of different length. Single, 3, 10, and 20-yr data were carefully merged into a decadal sequence. For, example bidecadal data were merged into 2 consecutive decades. Corrections for offsets in specific millennial period of time do not exceed 41 yr. Off- sets of laboratories from INTCAL98 are typically < 10 yr, except for the 4th and 5th millennia BC where a few offsets were greater (ca. 20 yr). We have already discussed hemispheric and regional offsets that can be greater than the above millennial offsets, for example, the 60-yr period from AD 1830 to 1890 for trees affected by upwelling caused by the Benguela current off western South Africa. Some “noise” is unavoidable in INTCAL98 but we believe that the signal processing technique discussed above will minimize that. 144 P E Damon, A N Peristykh

The Results of Spectral Analysis of Decadal Data Figure 3 presents the results of Fourier spectral analysis. The prominent peak at 88 yr is the Gleiss- berg cycle (Gleissberg 1944). We have shown its derivation from sunspot numbers in (Damon and Peristykh 1999). From Figure 1 it can be seen it is very prominent in this record of 9 millennia. We can be certain that its origin is solar. The cycle at 207 yr is inferred to be solar because of its relation to the Maunder Minimum of solar activity and other minima and maxima of 14C production (Damon and Sonett 1991; Stuiver and Braziunas 1993). There may be also a Sun-climate component as previously mentioned. This period seems to modulate the 88-yr Gleissberg cycle (Jirikowic 1993) as can be seen by the “beat” frequency (correspondent period is equal to 1/(1/88 – 1/207) ≈ 150 yr). The 207-yr cycle also has the second harmonic at 104 yr. The Gleissberg cycle appears to be amplitude modulated (AM) by the about 2050-yr Hallstattzeit, as manifested by the two equally spaced side peaks. The Hallstattzeit cycle is enigmatic (Damon and Sonett 1991). At least for the last three little ice ages in central Europe (Schmidt and Gruhle 1988) it appears to act as a gate with 600-yr high ∆14C variance preceded and followed by 1500 yr of low ∆14C variance (Damon and Jirikowic 1992). Although it produces symmetric side peaks by AM of Gleissberg cycle, such side bands are not observed on both sides of 207-yr cycle. The 207-yr cycle is always accompanied by a 226– 232 yr companion in various analyses but never with the significant 192 yr side peak as would be expected from AM by the Hallstattzeit cycle. This needs further investigation.

Figure 3 Fourier spectrum of detrended decadal ∆14C data. Note the intense 88-yr Gleissberg cycle with side bands.

Stuiver and Braziunas (1993) relate the 512-yr cycle to flux oscillations in the Atlantic Ocean thermohaline circulation. If this is valid perhaps the Hallstattzeit cycle period is also oceanic in origin from internally induced or externally forced oscillations. Pestiaux et al. (1988) have suggested that 10.3 ± 2.2, 4.7 ± 0.8, and 2.5 ± 0.5 kyr periods in the Maximum Entropy spectrum of the planktonic Application to Geophysics, Solar Physics, Astrophysics 145 and benthic foraminifera may be combination tones of the 41, 23, and 19 kyr periods in the insola- tion parameters. Their δ18O data covers the Last Ice Age. Two of the cores are from the northern Indian Ocean and a third is from the southern Indian Ocean. Stuiver et al. (1997) examined the δ18O record in the GISP2 ice core and found the strongest millennial change to be 1470 yr for the interval from 12,000 to 50,000 BP. One of the cores above is close to the west coast of India. Globorotalia menardii, a deeper water foraminifera from that core, is the only record showing a 1.5 kyr period in the δ18O record. The upper water foraminifera, Globigerinoides ruber, shows a δ18O peak at 2.6 kyr. Upper water foraminifera from the other 2 cores show periodicities of 2.3 and 2.7 kyr. Obviously, more work will be required to solve this puzzle.

Hyperfine Structure of the ∆14C Secular Variations The Nyquist frequency cutoff for decadal data is 0.05 yr−1, i.e. periods only longer than 20 yr can be detected by Fourier analysis. The INTCAL98 data has involved accommodation of 20 yr data. Con- sequently, the resolution is lower than 20 yr. Even with single-year data, we are limited by the carbon cycle low-pass filtering. The atmosphere contains an amount of 14C equal to a little more than 100 times the current steady state production rate. Consequently, an increase in production rate of 10% during one year would only result in an increment of 1‰ in atmospheric ∆14C. Thus, the atmosphere acts as a low-pass filter such that the shorter the period of a cycle the stronger the attenuation. The attenuation for an 11-yr cycle is about 70, whereas the change in production rate is about ±10%, consequently the atmospheric ∆14C variation would be about ±1.4‰. Obviously annual data of high precision are required to observe an atmospheric 14C cycle of this magnitude. Stuiver and Braziunas (1998) report measurement with a precision of < 2‰ of 48 single tree rings dendrochronologically dated from 1897–1994. After removing the trend with a cubic spline the average amplitude of the measured about 11 yr (Schwabe) cycle was ± 1.25‰ with an average lag time of 1.8 yr in reasonable agreement with theoretical expectation. Damon et al. (1998) are studying the Medieval Solar Maximum that occurs between the Oort and Wolf Minima. It seems to be the closest analog to the Contemporaneous Solar Maximum (Damon and Jirikowic 1994). Measurements are complete between AD 1065 to AD 1300 with a gap remaining between AD 1200 and AD 1238. The gap occurs because of the difficulty of obtaining samples of adequate weight from very thin tree rings that occur in that time interval. We expect to close the gap this winter. Figure 4 shows the results of Fourier analysis consisting of a Discrete Fourier Trans- form (DFT) of the detrended data from AD 1065 to AD 1200. The Hale cycle occurs at 21.3 yr and the Schwabe cycle at 10.4 yr with side peaks resulting from modulation by the 88 yr Gleissberg cycle. Four harmonics are also present. As previously mentioned, Lingenfelter and Ramaty (1970) at the 12th Nobel Symposium discussed the production of atmospheric 14C by supernovae explosions. The detection of such events required high precision and single-year dendrochronologically dated tree rings. Such a record became available in 1981 with a slight revision in 1998 (Stuiver and Braziunas 1993; Stuiver et al. 1998). We searched the data that extended back to AD 1510 and found no evidence for the Tycho Brahe (1572) and Kepler (1604) Supernovae in the data. This is not surprising considering the distance from the Earth of these events (10,000 and 26,000 light-years). For this reason we chose SN1006AD for a pilot study (Damon et al. 1995) because it exploded at a distance of 4000 light-years. Up until recently, it was thought to be the closest supernova to Earth. Recently, a young nearby supernova X-ray remnant γ 44 has been discovered (Aschenbach 1998). -rays from Ti (T1/2 = 47 yr) have been observed (Iyudin et al. 1998). Its distance from Earth is estimated to be only about 650 light-years. SN1006AD has been observed to have X-ray, optical, and radio remnants. The remnant is in the form of a shell and it 146 P E Damon, A N Peristykh

Figure 4 Spectral analysis of single-year data from AD 1065 to AD 1200. The data are from a high-precision (<2‰) record obtained with state-of-the-art beta counting. is a known pulsar. According to theory, acceleration takes place within the remnant at some time after the incandescent explosion producing pions that decay to two hard (GeV) γ-rays (Berezinskii and Ginzburg 1990). Upon reaching the Earth, the γ-rays initiate electron-positron showers. The electrons and positrons have near collisions with atmospheric nuclei emitting bremsstrahlung radiation (γ-rays). The bremsstrahlung γ-rays are ultimately degraded until they fall into a “giant” γ-n, γ-2n cross-section between 10 and 40 MeV. The emitted neutrons are fast but about two thirds are thermal- 14 14 ized producing C that oxidizes to CO2 and participates in the C–O cycle. We measured the cellulose in tree rings before and after the arrival of light from SN1006AD and have found a distinct pulse (Figure 5) beginning after AD 1008 with a very fast rise time to a maximum between AD 1011–1015. Following AD 1015 the pulse decays as it equilibrates with the biosphere and ocean mixed layer. By AD 1021, ∆14C starts rising to the Oort ∆14C Maximum, which occurs a little over 20 yr later. The peak height of SN1006AD event is 8‰. A ∆14C event from a single enormous solar flare would have a fast rise time but would begin to decay sooner than after the 4-yr maximum. A series of solar flares would not have as fast a rise time. We suggest that this is a unique ∆14C event and no coincidence that it begins only 2 yr after the light from SN1006AD first arrived on 30 April 1006 (Murdin 1985). Application to Geophysics, Solar Physics, Astrophysics 147

Figure 5 ∆14C pulse due to γ-ray arrival following the supernova first observed optically on 30 April 1006 AD. Note the very fast rise time (only 3 yr to maximum level). The 14C data were obtained by AMS at ±3‰ precision.

CONCLUSION We have come a long way since the fifth Radiocarbon Conference held at the University of Cam- bridge in 1962. It was at this conference that the 14C community first became aware of the need for calibration of the 14C time scale. Thirty-eight years later, after much labor by dendrochronologists and radiocarbon calibrators, we now have a calibration based on analysis of tree rings that reaches back 11,854 yr. All 14C measurements have been obtained by high-precision beta counting. A combination of varve data and U-Th and AMS measurements on coral carry the calibration back to 15,000 BP. Preceding 15,000 BP, linear approximations stretch the coral calibration back to 24,000 BP. More work on coral calibration is of the utmost importance not only for dating but also for a better understanding of the climate record and the role of climate in the carbon cycle as wit- nessed by the inverse correlation between ∆14C and δ18O during the Bølling and Younger Dryas involving large changes in both ∆14C and δ18O. We restricted the use of INTCAL98 for Fourier analysis to the period from 7195 BC to avoid large gaps in the record (20–50 yr) prior to 7195 BC. Hopefully, it will be possible to fill these gaps. For other reasons discussed, the INTCAL98 data is not ideal for Fourier analysis. Nevertheless, the results are quite gratifying as can be seen in Figure 3. There is information in the hyperfine structure of the ∆14C variations that can only be observed in high-precision single-year data. This includes high frequency components of the solar cycle as can 148 P E Damon, A N Peristykh

be seen in Figure 4. Such data can reveal information concerning the arrival of γ-rays from a supernovae explosion as shown in Figure 5. Also, the field of space physics, the study of radiation disturbances in space which is referred to as space weather, is becoming increasingly important. For example, a single solar flare can shut down an electrical grid, cause losing track of a fleet of Earth orbiting satellites or even cause the death of an astronaut outside the shelter of the Earth’s magnetic field. If there were, as we believe, giant solar flares or sets of solar flares in the past that can be detected in 14C records (Damon et al. 1989; Peristykh and Damon 1995, 1999), single-year data will be required. Moreover, it appears that coastal upwelling of deep ocean water releasing nutrients and 14C depleted 14 CO2 is time dependent. Times of extremely high upwelling, sufficient to change the C content of a prevailing air mass, could be dampened in decadal or bidecadal data. Ekman transport is wind driven and so interesting climate changes could be missed. In addition, single-year data can be merged into decadal data yielding increased precision.

ACKNOWLEDGMENTS This paper has received support from NSF Grants ATM-9819228 and EAR-9730699, and the Uni- versity of Arizona.

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RADIOCARBON BEYOND THIS WORLD

A J Timothy Jull1 • Devendra Lal2 • George S Burr1 • Philip A Bland 3 • Alexander W R Bevan4 • J Warren Beck1

ABSTRACT. In this paper, we review the production of radiocarbon and other radionuclides in extraterrestrial materials. This radioactivity can be produced by the effects of solar and galactic cosmic rays on solid material in space. In addition, direct implantation at the lunar surface of 14C and other radionuclides can occur. The level of 14C and other radionuclides in a meteorite can be used to determine its residence time on the Earth’s surface, or “terrestrial age”. 14C provides the best tool for estimating terrestrial ages of meteorites collected in desert environments. Age control allows us to understand the time constraints on processes by which meteorites are weathered, as well as mean storage times. Third, we discuss the use of the difference in 14C/12C ratio of organic material and carbonates produced on other planetary objects and terrestrial material. These differences can be used to assess the importance of distinguishing primary material formed on the parent body from secondary alteration of meteoritic material after it lands on the earth.

INTRODUCTION Cosmic rays interact with all solid objects in the solar system to produce radioactivity, from dust grains and meteorites to planetary bodies. Besides the well-known production of radiocarbon in the terrestrial atmosphere, spallation reactions of galactic (GCR) and solar (SCR) cosmic-ray particles on oxygen and silicon, and some other target elements result in the production of many radionuclides. This radioactivity produced in space can give us important information on the variations of GCR, SCR and also energetic particles emitted by the Sun. These high-energy reactions are different from the atmospheric production of 14C by thermal neutron effects on nitrogen. In this paper, we review several aspects of extraterrestrial or “cosmogenic” 14C. The production of this radionuclide in space can be used for several important applications, which we will discuss in this paper, specifically: 1. Lunar samples. We can use the levels of cosmic-ray-produced 14C in lunar samples, to estimate the effects of SCR and GCR production and the possibility of fluctuations of SCR in the past. 2. Terrestrial ages of meteorites. We can use the 14C produced in space as a method to measure the terrestrial residence time (terrestrial age) of meteorites after they fall to earth. The level produced in space is used as the “zero age” and one can then calculate the terrestrial age from the amount of 14C remaining. 3. Contamination studies of meteorites. A third application of considerable interest is useful in the determination the relative amounts of terrestrial contamination in organic compounds found in some meteorites. Because of the great interest in Martian meteorites that was generated by the work of McKay et al. (1996), a way of distinguishing between indigenous organic material and contamination is necessary. In this case, we can use the difference between 14C produced in space and the levels produced in the terrestrial atmosphere to identify different sources of carbon in these samples. Recent carbon produced at the surface of the earth from atmospheric sources will have a much different signature than spallogenic carbon produced in organic or car-

1NSF-Arizona AMS Laboratory, University of Arizona, 1118 East Fourth St., Tucson, Arizona 85721, USA. E-mail: [email protected]. 2Scripps Institution of Oceanography, Geological Research Division, University of California San Diego, La Jolla, California 92093, USA 3Natural History Museum, Cromwell Road, London SW7 5DB, England 4Western Australian Museum, Francis St., Perth, WA 6000, Australia

151 152 A J T Jull et al.

bonate materials in space. Hence, these differences can be used as a marker of origin of this carbon and allow us to distinguish these components. In this paper we will review these three important applications of 14C in extraterrestrial materials and give some examples of their applications.

RADIOCARBON IN LUNAR SAMPLES Our first example is 14C produced on the surface of the Moon. Lunar materials, both soils and rock surfaces provide long records of continuous production of radionuclides by GCR and SCR spallation. If we can obtain the records of different radionuclides that integrate different periods of time, we can estimate cosmic-ray intensities and variations of SCR fluxes in the past (see Reedy 1980; Reedy and Marti 1991) and possibly also differences in spectral shape. Fink et al. (1998) and Jull et al. (1998a) have summarized results of many radionuclides that have been used to determine variations of SCR fluxes in the past. Several mean lives of the radionuclide, i.e. for 14C about 20–30 ka of continuous exposure is required to reach a saturation level, as this cosmic-ray-produced 14C will build up according to an exponential increase as shown in Equation 1

P −λ N =−()1 e t 14 λ . (1)

Here, the production rate, P, is a combination of production from both SCR and GCR. N14 is the number of 14C atoms, λ is the 14C decay constant, and t is time. The production rate is dependent on the depth of the sample in the rock or core and geometry. The build-up time for 14C is much less than the exposure times at the lunar surface (which are in the millions of years, estimated from long-lived radionuclides) and also erosion or gardening by micrometeorite impacts. These processes occur on a time-scale of a few millimeters per million years (Langevin et al. 1982) and do not disturb 14C, but they are important in the understanding of longer-lived nuclide distributions. In the past, some 14C work was done with gas counters, on samples from the top few centimeters of lunar rocks 12002 (Boeckl 1972) and 12053 (Begemann et al. 1972) and these suggested high levels at the very surface. These early counter-measurements data had relatively large uncertainties due to the size of the samples studied. Boeckl (1972) used the high surface 14C values as evidence for an enhanced solar- 2 proton 4π flux of 200 protons/cm /s (Ep>10 MeV; R0=100 MV) over the last ~10 ka. Further, Bege- mann et al. (1972) had suggested that the very surface layer of lunar samples could be implanted with solar-wind 14C, and this could account for the enhanced surface activity in rock 12002. Fireman (1977) also studied surface enrichments which he ascribed to solar-wind implantation of 14C. Measurements on finer slices of rock had to wait for the development of accelerator mass spectrometry (AMS) and only in the last decade have the small size requirements of AMS measurements allowed studies of the production rate and depth dependence of 14C in millimeter-sized slices of lunar rocks (see Jull et al. 1992, 1998a). Previous work with 14C in lunar samples has identified three extraterrestrial sources of the 14C observed in lunar samples—production by nuclear reactions induced by GCR or SCR particles or implantation from either the solar wind (SW) or solar energetic particles (SEP). 1. Production of 14C by GCR in extraterrestrial materials. Armstrong and Alsmiller (1971) and Reedy and Arnold (1972) developed models for calculation of the production of 14C by cosmic- ray effects in the lunar surface. For a long time, models for the GCR production of 14C, based on Radiocarbon Beyond this World 153

the work of Reedy and Arnold (1972) did not result in a good fit to lunar samples. Hence, Born (1973) and Rao et al. (1994) had to increase calculated production rates to fit their experimental data. Over the years, cross sections have been revised and improved using AMS studies of arti- ficially-irradiated foils (e.g. Jull et al. 1989a, 1998a; Sisterson et al. 1997a, 1997b, 1997c). A new model by Masarik and Reedy (1994) was used to calculate production rates of 14C by GCR particles in the meteorite Knyahinya1 (L5 chondrite) as a function of depth (Jull et al. 1994). This model now gives good agreement (~10%) with measured values for both meteorites and lunar samples (see inter alia, Jull et al. 1998b; Wieler et al. 1996). Figure 1 shows some sample production rates for a typical meteoroid of different sizes irradiated in space.

Figure 1 Depth dependence of GCR production of 14C in meteoroids of different radii (from Wieler et al. 1996). The horizontal line is the mean value observed for H6 falls, with the dashed lines showing the range observed.

2. SCR Effects. Solar-cosmic-ray particles, >98% protons, with energies of tens to hundreds of MeV, have a range of ~1 cm in rocks. The SCR flux can be approximated (Reedy and Arnold 1972) as a distribution in rigidity units of the form

dJ/dR= k exp (–R/R0) ( 2 )

where J is the flux, R is the rigidity (pc/Ze) of the particles, R0 is a spectral shape parameter, given in units of megavolts (MV), and k is a constant. Fluxes for SCR are quoted for proton 2 energies >10 MeV and for a 4π solid angle, defined as J10, and has units of protons/cm /s. Val- ues of R0 in the range 70 to 125 MV have been fitted to a variety of radionuclide data from

1Meteorites are named by the Nomenclature Committee of the Meteoritical Society and reported in the “Meteoritical Bulle- tin”, published in Meteoritics and Planetary Science (e.g. Grossman 1999). Traditionally, meteorites are named for the near- est post office or named geologic feature. Due to the proliferation of meteorites from desert regions and Antarctica, other more generic names and numbers are often assigned to these meteorites. 154 A J T Jull et al.

lunar samples (see for example Reedy and Marti 1991; Rao et al. 1994; Fink et al. 1998; Jull et al. 1998a). SCR effects in meteorites are usually not observed since the outer surface of the meteoroid is ablated during atmospheric entry. The best example of SCR production is 26Al in the Salem meteorite (Nishiizumi et al. 1990). There has been no measurement of excess 14C attributed to SCR in meteorites. One interesting question is whether this spectral shape is actually a good model for SCR at all energies (Lingenfelter and Hudson 1980). 3. Implanted energetic particles: The solar wind is a stream of particles, mostly protons, emitted by the Sun with an average flux at 1AU of ~2 × 108 protons/cm2/s (Keays et al. 1970) and ~1 keV/amu in energy. The range of such particles in rock is ~30–40 nm in rock (Ziegler et al. 1989). There is also a possibility of implanted of higher-energy (up to 10’s of MeV) particles emitted during solar flares (e.g. Wieler et al. 1996; Nishiizumi et al. 1997). As already mentioned, Begemann et al. (1972) first suggested that the very surface layer of lunar samples could be implanted with solar 14C. Fireman et al. (1976, 1977) found higher than expected levels of 14C in the 600–1000 °C temperature fractions of the Apollo 11 soil 10084 and Apollo 17 trench soils 73221, 73241 and 73261, which they interpreted as implanted solar wind 14C. If implanted from the solar wind and not another source, this result would imply a 14C/1H ratio in the solar wind of ~5 × 10−11. Jull et al. (1995a) reported on an experiment that confirmed the existence of this implanted 14C component and concluded that there was an implanted 14C flux on the very surfaces of lunar rocks and soil of 4 to 7 × 10−6 14C/cm2/s, which is equivalent to a 2–3.5 × 10−14 14C/H ratio. However, in this work the possibility of sources other than just the solar wind were considered. These results suggested that this was good evidence for an implanted 14C component in the surface soil and rock.

Laboratory Procedures for 14C Extraction from Lunar Samples and Meteorites Cosmogenic 14C can be extracted from meteorites, lunar rocks, or soils by fusion of the rock powder with iron (which is used to enhance combustion) in an oxidizing environment. Lunar samples are crushed, if not already a powder, and several grams of iron chips are added to enhance combustion. The meteorite samples were crushed to a powder, weighed, and treated with 100% phosphoric acid to remove carbonates from the material. In addition, the samples are preheated to 500 °C in air to remove contaminants due to organics and adsorbed CO2. The gas from the acid etching was collected and the 14C ages of this material was measured. The residue was washed with distilled water and dried. This material was then placed in an alumina crucible and mixed with about 5 g of iron chips used as a combustion accelerator. The crucible is placed in an oven at 500 °C for 1 hr. The crucible is then placed in an RF furnace and heated to melting in a flow of oxygen. Any evolved gases are passed over MnO2 to remove sulfur compounds and CuO/Pt at 450 °C to oxidize all carbonaceous gases such as CO and CH4 to CO2. The volume of the gas is measured, and the gas is diluted 3 14 with about 0.5–2.5 cm of C-free CO2, as a carrier. This gas is finally converted to graphite powder over an Fe catalyst, which is then pressed into an AMS target holder. The target is mounted in a 32- position wheel in the AMS ion source and the sample 14C/13C is compared to that in known NIST standards. Procedures for the AMS analyses have been reported by Jull et al. (1990, 1993a) and details of the calculations by Donahue et al. (1990). Samples of graphite as small as 100 µg are then analyzed by AMS. The basic studies on Bruderheim, other meteorites, and blank rock samples were published in Jull et al. (1989b, 1993a, 1994, 1998b).

14C in a Lunar Rock Surface In Jull et al. (1998a), we reported on 14C in a series of samples from lunar rock 68815, collected from the Apollo 16 site. With a rock, we should expect to obtain a complete SCR profile indepen- Radiocarbon Beyond this World 155 dent of any gardening or loss of material which we might expect in a soil core top. Erosion by micrometeorites, of the order of mm/million years, ought not to affect 14C. Rock 68815 was removed from the top of a large boulder at the Apollo 16 site (see Figure 2 in Color Plate 3.). Results of our measurements are shown in Figure 3, from a surface value of about 66 dpm/kg. The surface profile which shows a decline of 14C to about a depth of 3g/cm2, due to SCR production of 14C as well as GCR. The subsequent increase to about 50 g/cm2 depth is due to GCR production alone. The results show that our measurements give a similar trend of 14C with depth in the rock as observed previously in Apollo 12 lunar rocks, numbered 12053 and 12002, by Begemann et al. (1972) and Boeckl (1972), but with more detail. We also studied two very-surface patina samples that covered a larger area of rock, which gave a similar result. The results do not show the very high surface value observed by Jull et al. (1995a); we shall return to this observation later.

Figure 3 Depth dependence of 14C due to SCR and GCR production in the surface of lunar rock 68815

The measurements for 14C production in 68815 as a function of depth are illustrated in Figure 3. Jull 2 et al. (1998a) reported the best fits to the data for R0 of 115 MV and J10 of 103 protons/cm /s. How- ever, they also found other reasonable fits for different values of R0 from 100 to 130 MV, with equiv- 2 alent values of J10 of 130 to 88p/cm /s, since there are a series of solutions for both J10 and R0 which fit this spectrum. However, all fits gave a flux of 19 p/cm2/s for J (E>57MeV). The best fits were found for spectra with the model energy distribution used by Reedy and Arnold (1972). The best fit R0 was that with the smallest standard deviation of the observed/calculated ratios of SCR activities using calculated SCR production rates for a wide range of Ro values. The average ratio for the best-fit Ro was then used to adjust the arbitrary solar-proton flux used in the calculation to get the best-fit flux. For the Apollo 15 soil cores, Jull et al. (1998a) reported the best fit used the data between 0.8 and 5 g/cm2 with the 14C activity of the surface sample being significantly lower than 156 A J T Jull et al. calculated. Hence, we can fit most data from rock 68815 and the soil cores to the calculated SCR profiles. If there is an implanted component, as discussed by Jull et al. (1995a), this could affect the observations on the very surface.

Table 1 Radionuclides used in extraterrestrial studies Nuclide Half-life References 14C 5.73 ka Begemann et al. (1972); Boeckl (1972); Jull et al. (1989b, 1995a, 1995b) 59Ni 76 ka Lanzerotti et al. (1973) 41Ca ~100 ka Fink et al. (1998) 81Kr 229 ka Reedy and Marti (1991) 36Cl 300 ka Nishiizumi et al. (1989, 1995) 26Al 700 ka Kohl et al. (1978); Nishiizumi et al. (1990, 1995); Fink et al. (1998) 10Be 1.5 Ma Nishiizumi et al. (1988, 1990, 1995, 1997); Fink et al. (1998) 53Mn 3.7 Ma Kohl et al. (1978); Nishiizumi et al. (1990) 21Ne, 22Ne, 38Ar Stable Rao et al. (1994)

Table 2 Solar-proton spectral parameters and 4π integral fluxes (p/cm2/s) above various energies (in MeV) determined from radionuclides in lunar samples (adapted from Jull et al. 1998a)

Time range Nuclide References R0 (MV) E>10 E>30 E>60 1954–1964 22Na, 55Fe Reedy (1977) 100 378 136 59 2 × 104 yr 14C Jull et al. (1998a) 110–115 103 ± 5 42 17 1 × 105 yr 41Caa Klein et al. (1990) 70 120 28 7 Fink et al. (1998) 80 200 56 16 3 × 105 yr 81Kra Reedy and Marti (1991) ~85 ——14 5 × 105 yr 36Cla Nishiizumi et al. (1995) ~75 100 26 7 1 × 106 yr 26Al2 Kohl et al. (1978) 100 70 25 9 1 × 106 yr 10Be,26Alb Nishiizumi et al. (1995) 75 100 26 7 10Be,26Al Michel et al. (1996) 125 55 24 11 Fink et al. (1998) 100 89 32 12 2 × 106 yr 10Be,26Alb Nishiizumi et al. (1988) >70 — ~35 ~8 6 × 107 yr 53Mn Kohl et al. (1978) 100 70 25 9 ~2 × 106 yrc 21,22Ne, 38Ar Rao et al. (1994) 80–90 58–87 ~22 ~7 aThe values obtained for 41Ca, 36Cl, and 81Kr (in italics) are uncertain due to few cross-section measurements. bThe determinations for 26Al before 1996 are based on old cross sections and could change using recently measured cross sections (Michel et al. 1996; Sisterson et al. 1997c). cThe determinations based on stable nuclides depends on the erosion rate model used by Rao et al. (1994), who assumed an erosion rate of 1–2 mm/Ma and the estimate exposure age of ~2 Ma based on 10Be in 68815 (Nishiizumi et al. 1988).

Variations in the SCR Flux

In Table 2, we list for comparison some R0 and flux estimates which were obtained from measurements of several long-lived and stable isotopes in lunar rocks (see Jull et al. 1998a). The values of 14 R0 and flux obtained in the work from C measurements are usually larger than the equivalent values from studies using longer-lived nuclides, except 41Ca. In addition, for some nuclides, cross sections are not sufficiently well-known to allow us to determine uniquely both R0 and J10. However, 14 the fact that the rigidity parameter R0 and fluxes for the relatively short-lived C (Jull et al. 1998a) and 41Ca (Fink et al. 1998) are higher than other nuclides, indicates to us that the SCR flux over the last ~10,000 to 100,000 yr must have been greater than for longer time-periods. This is particularly Radiocarbon Beyond this World 157 clear for the E>60MeV particles. To summarize the best fits of SCR 14C, this was found for a higher 2 R0 of 110 to 115 MV and a J10 flux of 108-98 protons/cm /s, respectively.

Million-year time scales from 53Mn, 10Be and 26Al studies. The first estimates of the SCR flux had 2 53 26 determined a value of J10 of ~70 protons/cm /s from Mn and Al measurements in rock 68815, with a spectral shape parameter R0 of 100 MV (Kohl et al. 1978). However, these authors also noted that this combination of flux (>10 MeV), R0, and erosion rates was not a unique solution. Later, 10 Nishiizumi et al. (1988) measured Be in lunar rock 68815, and deduced a higher flux at lower R0, using this radionuclide. This cast some doubt on existing cross section measurements. Using recent measurements of cross sections for the production of 10Be by protons (Sisterson et al. 1997a,b,c; Bodemann et al. 1993; Schiekel et al. 1996), a production rate for 10Be in the surface layer of 68815 to be ~25% higher than those calculated using earlier estimates can be determined. This resulted in a 2 value for R0=70 MV and J10 of ~100 protons/cm /s (see Jull et al. 1998a). The latest cross sections for 10Be and 26Al were used by Fink et al. (1998) in another lunar rock, 74275. Also recently, Michel et 2 10 26 al. (1996) estimated J10 of 55 protons/cm /s for R0=125 MV from Be and Al cross sections and reported profiles. The studies by Fink et al. (1998) and Nishiizumi et al. (1995) give J10 values within 14 14 ~15% of our estimates of flux from C, but not for the same R0. Several radionuclides, including C, are produced by reactions with threshold energies of 30 to 60 MeV. Thus, radioisotopes having a half- 6 lives longer than 10 yr generally appear to have lower J10 and lower R0 than our new estimates based on the shorter-lived 14C. This is true for the results of Kohl et al. (1978), Rao et al. (1994), Michel et al. (1996), and Fink et al. (1998) compared to the 14C work of Jull et al. (1998a).

The last 300,000 yr using 81Kr, 41Ca and 14C studies. The radionuclide 81Kr was measured in 68815 for E>60MeV, by Reedy and Marti (1991), who derived a higher J10 that discussed in the previous section. Together with 41Ca (Fink et al. 1998) data on 74275, this appears to be in better agreement with the 14C data from Jull et al. (1998) than the longer-lived nuclides. Some uncertainties in cross sections remain for those two radionuclides. Klein et al. (1990) examined 41Ca in rock 74275 2 and calculated an SCR flux of 120 protons/cm /s but with a low R0 of about 70 MV. Recent calcu- 41 2 lations which fitted the Ca measurements on 74275 suggested a J10, of ~200 protons/cm /s and R0 of about 80 MV should give a good fit (Fink et al. 1998). In addition, the work of Fink et al. (1998) shows a higher flux than the longer-lived nuclides and confirms the suggestion of enhanced SCR fluxes in the last ~100 to 200 kyr. So far, 36Cl has not proved useful for lunar studies. Short-lived radioisotopes 22Na and 55Fe also show higher estimated solar-proton fluxes, as these nuclides are greatly affected by the very high solar activity in the decade immediately before the recovery of the Apollo samples (Reedy 1977). Indeed, we could state that all values of J10 and R0 estimated for all radionuclides are within the wide observed range of spacecraft measurements. Spacecraft results 2 2 depend strongly on the solar cycle and show variations from 63 p/cm /s (R0=40MV) to 312 p/cm /s (R0~70MV), and have been discussed in detail by Reedy (1980, 1996) and Goswami et al. (1988).

Implantation Effects Jull et al. (1995b, 2000a) studied an apparent surface enrichment of 14C, especially in lunar soil grains. The flux of H from the solar wind of ~2 × 108 H/cm2/s. In order to compare the flux of implanted 14C with H we need to be able to estimate the surface area of the grains. The limits for 14C/ H estimated by Jull et al. (2000a) were ~0.4 to 0.8 × 10 −14, which was based on studies on grain-size separates of Apollo 11 soils and also Apollo 16 soil 64501. A deep soil sample collected from Apollo 17 was used as a control. The quoted ratio is lower than the ratio of 2.2 to 3.5 × 10−14 for the same value previously reported by Jull et al. (1995b) based on etchings of bulk 10084 soil and also soils 73221, 73241, and 73261, from the Apollo 17 trench soils. The original study of Fireman 158 A J T Jull et al.

(1977) measured similar surface enrichments of 14C to Jull et al. (1995b), but estimated a much higher ratio of 14C/H of ~10−11, due to different assumptions about the surface areas of the samples.

Other Information from 14C in Lunar Soil Cores Work on lunar core 15001-8 and rock 68815 (Jull et al. 1998) suggest a possible enhancement in the SCR flux in the time scale integrated by 14C. The very surface of lunar rocks and soil could also be affected by implantation of particles directly from the Sun, from the solar wind or solar flare events. Recent data (Nishiizumi et al. 1997) suggest the very surface layer might have be implanted with solar 14C and perhaps some 10Be. Recently, further experiments on lunar soils (Jull et al. 2000a) confirm the existence of such a component, but raised new questions about whether this could indeed be of solar-wind origin.

Figure 4 Solar-proton spectral parameters and 4π integral fluxes (p/cm2/s) above 10 MeV, determined from radionuclides in lunar samples

METEORITE STUDIES AND TERRESTRIAL AGES Meteorites fall equally all over the world and can be recovered from all parts of the globe (Halliday et al. 1989). The infall rate has been described as a function of mass where log N = a log M + b , (3) and where N is the number of meteorites which fall per 106 km2 per year, > mass M in grams. Hal- liday et al. (1989) determined the constants a and b to be −0.49 and −2.41 for M<1030 g, and −0.82 and −3.41 for M>1030 g, based on observations of meteoroids. This would result in an infall rate of M>10 g of 83 events per 106km2/yr, or roughly one event per km2 in 10,000 yr. The arid and polar regions of the world appear to be the best locations for storage of meteorites, where they can survive for long periods of time in such environments (Nishio and Annexstad 1980; Nishiizumi et al. 1989; Jull et al. 1990, 1993a, 1998b, 2000b). Large numbers of meteorites have been recovered from the arid and semi-arid regions of North Africa, Arabia, North America and Western Australia. One of the first recognized areas for collections of meteorites was Roosevelt Radiocarbon Beyond this World 159

County, New Mexico (Scott et al. 1986; Sipiera et al. 1987). The Nullarbor region of Australia and the northern Sahara Desert in Africa are also wonderful sources of meteorites (Wlotzka et al. 1995; Bevan et al. 1998; Schultz et al. 1998). Searches have been undertaken recently in the Namib desert and dry lakes in California, and less explored areas such as the remote deserts of southern Africa and South America may yet yield many more meteorites. The cold desert of Antarctica is also a large store of meteorites. Beginning in 1969, Japanese researchers recovered a number of meteorites from Antarctica. They have continued to recover meteorites annually. In 1976, Cassidy and Olsen undertook an expedition to Antarctica to recover meteorites from the Allan Hills blue icefield, located in easy range of the US base at McMurdo. This program has since developed into the US Antarctic Search for Meteorites (ANSMET), and over 17,400 meteorites have been recovered from Antarc- tica, approximately equally divided between US and Japanese collections (Grady 2000). For comparison, the number of meteorites recovered (Grady 2000) from deserts are 1508 for the Sahara Desert, and >280 for Nullarbor. The study of the terrestrial ages of these meteorites is of great utility, as it gives us information concerning the storage and weathering of meteorites and the study of fall times and terrestrial age. The 14 most useful for many meteorite collection areas is C (t1/2=5730 yr), as summarized by Jull et al. (1990, 1993, 1998). Early measurements by Suess and Wänke (1962) and Goel and Kohman (1962) on large meteorite samples (10–100 g) were made by 14C decay counting. Later, Boeckl (1972) used 14C to estimate terrestrial ages of some meteorites found in the central and southwestern USA, using ~10 g samples. Fireman (1978) and Kigoshi and Matsuda (1986) made some measurements on Antarctic meteorites using similar methods. The first measurements which used AMS for 14C terrestrial ages of Antarctic meteorites were by Brown et al. (1984). Subsequently, AMS has been almost exclusively used for 14C terrestrial age measurements using smaller sample sizes (0.1–0.7 g), mainly by the Arizona group, summarized by Jull et al. (1984, 1989b, 1990, 1993a, 1993b, 1994, 1995a). Some other measurements were done at Toronto, by Beukens et al. (1998) and Cresswell et al. (1993), and by a Ger- man-Swiss consortium (Neupert et al. 1997; Stelzner et al. 1999). Longer-lived isotopes like 81Kr (Freundel et al. 1986; Miura et al. 1993) and 36Cl (Nishiizumi et al. 1989) can also be used to determine longer terrestrial ages. This gives us information beyond the useful range of 14C of about 40,000 years. In the case of samples at the limit of 14C age determination, we can sometimes place upper limits on their age by a lower limit determined by the 36Cl age.

Production Rate of 14C and Interpretation of 14C Terrestrial Ages Jull et al. (1994) and Wieler et al. (1996) have discussed the variation in 14C production rate at different depths in meteorites of different sizes. Recent falls generally show activities of 14C equivalent to a production rate of 38–58 atoms/min/kg (as shown in Table 3). Wieler et al. (1996) showed calculations for meteorites of preatmospheric radii from 20 to 45 cm where the saturated activity (or production rate) should vary from about 38 to 52 dpm/kg for an H chondrite. Smaller objects have lower production rates of 14C. In Figure 1, we have shown the expected production rates for a sample recovered from a given depth for meteorites of H-chondrite composition of different sizes (Wieler et al. 1996). Measurements on the Knyahinya L-chondrite (R=45 cm) gave values of 37 at the surface to 58 dpm/ kg at the center of the meteorite. Nearly all 14C is produced from spallation of oxygen, with only about 3% produced from Si (Sisterson et al. 1994). Hence, normalization of the saturated activity observed to the oxygen content works well. We estimated the saturated activity for a given class of meteorite by normalizing the mean value of the 14C content of Bruderheim (51.1 dpm/kg) to the oxygen content of the meteorite determined from bulk chemistry or from average compositions (Mason 160 A J T Jull et al.

1979). The scatter in measurements on saturated falls suggests that an uncertainty of ±15% should be included in estimates of the terrestrial age to account for uncompensated shielding or depth effects, as well as experimental uncertainty (Jull et al. 1993a). This variation is confirmed by the study of the depth dependence of 14C in the chondrite Knyahinya (Jull et al. 1994), which has an estimated preatmospheric radius (R) of 45 cm. Some measured 14C activities from known meteorite falls are given in Table 3. Because many of the meteorites collected from Antarctica are small, <100g, compared to other finds, these require additional criteria to determine the production rate of 14C. This information is not available in all cases, but we use rare gas and other radioisotope (e.g. 26Al and 10Be) data to verify that the meteorite appears to have been irradiated as a body with a radius of 20–50 cm. We can also use 22Ne/21Ne ratios to estimate the shielding depth of the sample in a meteoroid, Schultz et al. (1996) have summarized the available Ne isotopic data. For apparently smaller objects, a lower saturated activity should be used. In many cases where we do not have sufficient information to make this determination, we will quote the result for the standard values listed in Table 1. We calculated the 14C activities in dpm/kg and the terrestrial ages as described by Jull et al. (1993a).

Table 3 Saturated activities measured in recently fallen meteorites Meteorite Type Year of fall 14C (dpm/kg) Reference Bruderheim L6 1962 51 ± 2 Jull et al. (1993a) Dhurumsala LL6 1860 55.7 ± 2.3 Stelzner et al (1999) Holbrook H6 1912 44 ± 1 Jull et al. (1998b) Peekskill L6 1992 51.1 ± 0.4 Graf et al. (1996) Torino H6 1992 42 ± 2 Wieler et al. (1996) Mbale L6 1996 58.1 ± 0.4 Jull et al. (1998b)

Desert Meteorites 14C provides the best method of estimating the terrestrial age of meteorites recovered from desert environments. When combined with good recovery statistics and weathering information, we can use these ages to assist in determining infall rates (Bland et al. 1996). For the Nullarbor Plain (West- ern Australia) and for some other locations, we observe an approximately exponential drop-off of number of meteorites with increasing terrestrial age (Bevan et al. 1998, 1999; Jull et al. 1995b). Jull et al. (1990, 2000b), Knauer et al. (1995), Neupert (1996), Neupert et al. (1997), Stelzner et al. (1999), and Wlotzka et al. (1995) have reported on terrestrial ages of meteorites from the Sahara desert in Libya and Algeria, which show similar trends. Different climatic regimes and local geology can affect the distribution of terrestrial ages of meteorites from areas such as the Sahara desert and Roosevelt County, as weathering occurs at different rates depending on sample chemistry and local climatic effects. Figure 5 (Color Plate 3) shows the age distribution of meteorites from some of these locations. In general, the Nullarbor and Sahara meteorites show an approximately exponential decrease of number of finds with terrestrial age to at least 30 ka. Since weathering gradually destroys meteorites, we expect that in a given population of finds, that the resulting distribution should show some exponential dependence on age. As an example, consider a collection where meteorites fell continuously directly on the collection area. The meteorites then should eventually disintegrate and reach a steady state where the disintegration rate will match the infall rate. Therefore, the number will decrease with increasing age, and so there should be more young meteorites than older ones. This is the expected distribution based on a simple first-order model of meteorite accumulation (Freundel et al. 1986; Jull et al. 1993a). We can show an example of the case of the Western Australian meteorites. Radiocarbon Beyond this World 161

Here, few meteorites beyond the range of 14C are observed and this profile shows the simple decay model for meteorite ages. Due to other effects, at many sites this simple relationship is not always observed. Many stony meteorites can survive in desert environments for long periods (Jull et al. 1990, 1995b; Bland et al. 1998). In a new approach to the question of meteorite survival and weathering, Bland et al. (1998) compared the terrestrial ages of many meteorites with the degrees of weathering observed. Previous studies (e.g. Wlotzka et al. 1995) had used a petrographic weathering estimate, however, Bland et al. (1998) introduced the idea of using Mössbauer spectroscopy, where the relative amounts of Fe in different valence states could be measured. Bland et al. (1996) further showed that meteorites of different composition weather at different rates, a fact known qualitatively to many meteoriticists, but difficult to quantify. Mössbauer provides that quantitation, and Bland et al. (1996) showed that H chondrites, which contain more metallic iron, weather at faster rates than L chondrites. We can also therefore conclude that achondrites, mainly basaltic rocks containing no iron, would survive the longest. Indeed there is some evidence to support this assertion in the observed distribution of terrestrial ages of Antarctic meteorites. This observation is particularly important as some interesting achondrites have also been recovered from these desert regions, although the vast majority are ordinary chondrites. Those of greatest interest are those of lunar and Martian origin which have been recovered from both cold and hot deserts.

14C–10Be Dating A limitation of using 14C for terrestrial age determinations is the need to make shielding corrections if the original meteoroid was very large or small (see Figure 1). One way to make these depth estimates is by 22Ne/21Ne ratios (Cressy and Bogard 1976). We can also use the production rate of a more long-lived nuclide such as 10Be to normalize the 14C production rate. Since both of these radionuclides are produced by the same spallation reactions on oxygen, their production in meteorites is almost always at a constant ratio. The 14C-10Be method allows us to correct for shielding effects, the only assumption we have is that the exposure age of the meteorite is sufficient to saturate 10Be (Jull et al. 1999b). This exposure age can also be verified by noble-gas data. Both 14C and 10Be can be measured by accelerator mass spectrometry (AMS) at the NSF-Arizona AMS Laboratory. The first attempt to apply this work was by Neupert et al. (1997, 1999), who correlated 14C, 26Al and 10Be in some Açfer meteorites, and used the results to estimate shielding-corrected terrestrial ages. Since both 14C and 10Be are produced by spallation of oxygen, their depth dependence is reasonably similar, and also the production ratio is independent of chemistry. This was demonstrated in the study of the 14C and 10Be depth dependence in Knyahinya, a large meteorite of some ~400 kg that fell in the Ukraine in the 19th century (Jull et al. 1994). An excellent example of the use of the 14C-10Be method for determining terrestrial age was shown by Kring et al. (2000) for the large fall of the Gold Basin meteorites. The Gold Basin meteorite shower was apparently due to the explosion of a large bolide (H4 chondrite composition) several meters in diameter over northwestern Arizona. Thou- sands of fragments of this object have been recovered by Kring and co-workers as well as by private individuals. We compared the results obtained for both of these radionuclides on splits of the same samples. By normalizing the 14C production rate to that of 10Be, we can correct for the problem of 14C production at a significant depth in a large object. The terrestrial age we determined is about 15,000 years (Kring et al. 1998, 2000) in the late Pleistocene. 162 A J T Jull et al.

Antarctic Meteorites Jull et al. (1998b) have discussed the terrestrial 14C ages of 95 meteorites collected from Antarctic blue ice fields by US scientists. A map showing locations of many of the main collection areas from which meteorites have been recovered is shown in Figure 6.

A — Sor Rondane Mountains (Asuka) MBR — Mount Baldr ALH — Allan Hills MET — Meteorite Hills B — Belgica Mountains MIL — Miller Range BOW — Bowden Neve OTT — Outpost Nunatak BTN — Bates Nunataks QUE — Queen Alexandra Range DOM — Dominion Range PAT — Patuxent Range DRP — Derrick Peak PCA — Pecora Escarpment EET — Elephant Moraine PGP — Purgatory Peak GEO — Geologists Range RKP — Reckling Peak GRO — Grosvenor Mountains STE — Stewart Hills HOW — Mt. Howe TIL — Thiel Mountains ILD — Inland Forts TYR — Taylor Glacier LAP — LaPaz Ice Field WIS — Wisconsin Range LEW — Lewis Cliff Y — Yamato Mountains MAC — MacAlpine Hills

Figure 6 Map of Antarctica, showing recovery locations of meteorites. Adapted from Antarctic Meteorite Newsletter. Radiocarbon Beyond this World 163

Because of low storage temperatures, we expect Antarctic meteorites to be stored for long periods of time in or on the ice. This is certainly confirmed by the results, which can easily be compared to the age distributions for non-polar sites and show longer residence times than observed in warmer desert environments. In Antarctica, we observe samples both within the range of 14C dating, up to 40–50 ka, and beyond. Nishiizumi et al. (1989), Cresswell et al. (1993), Jull et al. (1993b), and Michlovich et al. (1995) have shown that the age distributions of meteorites at the different Allan Hills icefields and Yamato collection sites in Antarctica can be very different. Nishiizumi et al. (1989) reported that many meteorites from the Allan Hills Main Icefield have long terrestrial ages, as determined by 36Cl (t1/2 = 301,000 yr). Indeed, two meteorites have been recovered, an H-chondrite from Allan Hills (ALH88019) and an L-chondrites from Lewis Cliff (LEW86360) which have very long terrestrial ages in excess of 2 Ma (Sherer et al. 1997; Welten et al. 1997). The largest single meteorite (with a terrestrial age >30 ka) is the 100 kg H5 meteorite LEW 85320 (Figure 8, in Color Plate 4).

Number 10

0 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 Terrestrial age (Kyr)

Allan Hills (Main) Yamato

Figure 7 Terrestrial age distributions of meteorites from the Allan Hills main icefield and the Yamato site. References cited in text.

Nishiizumi et al. (1989) summarized data on 67 meteorites from the Allan Hills Main Icefield, and they found most gave 36Cl terrestrial ages of chondrites of >100 ka, and up to 500 ka in a few cases. In Figure 7, we show a summary of terrestrial ages determined by 14C, 36Cl and 81Kr for meteorites from this icefield and also the Japanese Yamato site. Twenty Allan Hills main meteorites (~30%) were <70 ka, and from 14C, we found only five meteorites out of the 27 meteorites analyzed for 14C were <25 ka. The long terrestrial ages observed for the Allan Hills Main Icefield meteorites are normally explained by transport of the meteorites in flowing ice over large distances (Drewry 1985; Nishio and Annexstad 1980). By contrast, samples from other areas such as the Far Western Icefield (Figure 9, in Color Plate 5) and 14C ages from the Yamato site (Figure 10) show significantly younger population of meteorites. At these locations, most of the samples date within the last 40 ka, although the Japanese Yamato site does show a large range of terrestrial ages, with several meteorites of terrestrial age >200 ka (Nishiizumi et al. 1989; Michlovich et al. 1995). 164 A J T Jull et al.

In addition, some much older 81Kr ages on one class of achondrite meteorites (eucrites) up to 300 ka have been observed (Miura et al. 1993). This may indicate a special population group more resistant to weathering, or whether this is due to some uncertainties in the 81Kr production rate (cf. Reedy and Marti 1991). In the case of the Allan Hills Far Western icefield there are few meteorites with less than saturated 36Cl (Nishiizumi et al. 1989), indicating short terrestrial ages consistent with the observation of significant levels of 14C in most samples. These results are shown in Figure 9. We suppose meteorites at sites like the far Western icefield or Yamato cannot have been transported any significant distance in the ice, and most likely fell at the location where they were recovered. When pairing is also taken into account, these results allow us to deduce that the distribution of meteorite terrestrial ages can be related to ice flow patterns in the Allan Hills region.

16 14 12 10 8

Number 6 4 2 0 5 10 15 20 25 30 35 40 45 50 55 >60 Terrestrial Age (Kyr)

Figure 10 Terrestrial ages of meteorites from the Yamato site studied by 14C

We can summarize the age distributions from different icefield by listing the number with terrestrial ages <25 ka, and categorize sites by the number of such younger falls. This allows us to order icefields in terms of number of falls <25 ka as follows, based on 14C ages only (results are taken from Jull et al. (1993b, 1998b, 1999a); Cresswell et al. (1993); Michlovich et al. (1995), and other unpublished results from our laboratory). By contrast, the results for some desert meteorite collection areas show higher values, as seen in Table 4.

Table 4 Percentage of meteorites recovered from different locations with terrestrial ages <25,000 yr Location Percentage Acfer, Algeria 92 Roosevelt County, New Mexico, USA 45 Far Western Icefield, Allan Hills, Antarctica 65 Yamato Mountains, Antarctica 50 Middle Western Icefield, Allan Hills, Antarctica 30 Elephant Moraine, Antarctica 28 Allan Hills Main Icefield, Antarctica 7 Radiocarbon Beyond this World 165

Many Antarctic meteorites are clearly fragments of the same fall. Indeed, 2 very large shower falls, one of an H5 chondrite at Lewis Cliffs (Figure 8, in Color Plate 4) and another of an LL5 chondrite at Queen Alexandra Range, are well documented in the US Antarctic collection. Lindstrom and Score (1994) used statistical arguments to estimate that pairing might reduce the number of discrete Antarctic falls by as much as a factor of 2 to 6. They also suggested the frequent number of ordinary chondrites in some localities could indicate a pairing factor of as high as 5 for these meteorites. These estimates appear too high to be consistent with the terrestrial-age data available on the samples studied. However, it is possible the terrestrial-age results may have some selection bias against paired meteorites. If the high pairing were the case, any estimates of infall rates would be too high, as our calculations of infall rates rely on an estimate of the total number of meteorites collected in our to scale our observed age distributions to the whole population. We can compare the estimates of infall rate from different areas and the results summarized by Bland et al. (1996) in Table 5. Here we can see that the estimates from Allan Hills are not in disagreement with other estimates. It would be difficult to make such infall-rate estimates without any information on the fall times (terrestrial ages of the meteorites) and this demonstrates the utility of this information.

Table 5 Estimates of weathering and infall rates λ N (>10 g) Location (ka−1) (106 km2 yr−1) Reference Meteor observations 83 Halliday et al. (1989) Roosevelt County 0.032 116 Bland et al. (1996); Jull et al. (1991) Nullarbor Plain 0.024 36 Bland et al. (1996) Sahara Desert 0.011 95–431 Bland et al. (1996) Allan Hills Main ~50 Jull et al. (1998b) Allan Hills Far Western 0.017 53 Jull et al. (1998b)

USING 14C TO TRACE THE ORIGIN OF ORGANIC MATERIALS IN MARTIAN METEORITES McKay et al. (1996) reported evidence suggesting the possibility of biogenic fossils in the orthopy- roxenite meteorite, Allan Hills 84001. This discovery ignited a debate on the question of whether early Mars could have supported life. This meteorite, together with Elephant Moraine 79001 and other Martian (sometimes referred to as SNC) meteorites, form a class of rare meteorites called achondrites. These are basically igneous rocks and were apparently ejected from the surface of Mars (McSween 1994). The work of McKay et al. (1996) has stimulated much discussion as to the nature and origin of organic material in ALH 84001 and EETA 79001 in particular, and other Martian meteorites in general. Important clues to the origin of the organic material can be obtained from the amounts of 14C and the relative amounts of 13C/12C.

δ13 δ18 14 We have studied the isotopic ratios C, O and the C compositions of CO2 released from carbonates (Jull et al. 1997) and organic compounds in these meteorites (Jull et al. 1998c). We can use 14 δ13 C as well as their stable isotopic composition ( C) to identify their origin. Small amounts of CO2 produced by combustion in oxygen is let into a mass spectrometer to determined δ13C. The gas is then recovered from the mass spectrometer using liquid nitrogen to free out the gas into a glass ampoule and the gas in transferred to another system. Here, the sample is reduced to graphite and the 14C content of the sample is determined by AMS. Terrestrial organic material, which mainly origi- nates from biogenic activity, has levels of 14C consistent with the time since removal from equilibrium with the terrestrial biosphere or atmosphere, as discussed by Jull et al. (1998c, 1999b). 166 A J T Jull et al.

We can also estimate the effects on organic material exposed to radiation in space, might sometimes contain excess 14C produced by the action of cosmic rays. We know that meteorites are bombarded by cosmic rays in space. The 14C can be produced only by thermal neutrons on 14N, or to a much lesser extent by neutron capture on 13C. As we know that all samples of these meteorites so far recovered were irradiated as small objects in space with only trace water content, very few cosmic- ray generated neutrons can have been produced. Spergel et al. (1986) had shown that for objects of radius less than ~50 g/cm2 (about 150 cm in a rock, or approximately 19 kg in mass) that the cosmic ray induced thermal neutron flux is extremely small and neutron products are not detectable. For these meteorites, ALH84001, Nakhla and EETA79001 were much smaller than this size (2.1, 10, and 7.9 kg recovered mass, respectively), and thus the thermal neutron flux would be even, hence we can eliminate this production of 14C in the organic components of these Martian meteorites. We performed stepped heating experiments on several sample of these Martian meteorites. For EETA79001, the intermediate temperature (~400–600 °C) fractions reveal that the carbonate mineral component of EETA79001 has exchanged with terrestrial carbon dioxide to some extent, because we observe 14C in these fractions. However, the carbonate fraction of ALH84001 is low in 14C and this is consistent with an extraterrestrial origin. The large difference in both 14C and δ13C measurements of organic and carbonate fractions of ALH84001 indicates that it is extremely unlikely that they could have formed from a common reservoir of carbon. Also, we know from the work of Nyquist et al. (1999) that the carbonates likely formed 3.9 Gyr ago. Hence, if the organic material and carbonate coexist and were formed from a common reservoir, we would have to find a mechanism to produce the large differences in δ13C. If we only had the δ13C, we know that isotopic fractionation between methane and carbon dioxide can be large (about 70‰) at low temperatures can occur, but only under certain conditions. This would require that the 14C concentration (which is corrected for isotopic effects) in both phases be the same. Thus, the 14C “label” is very important and it tells us if these phases have some terrestrial contaminants which were either biogenic or at some time in equilibrium 14 14 with the atmosphere. C measurements reveals that the carbonate CO2 is low in C. The fraction 500–600 °C appears to be the purest carbonate release, with Fm (14C)<0.04. This is consistent with previous work on carbonates with δ13C of +40‰ (Jull et al. 1997) and our estimated 14C composition of carbonates irradiated in space. This also supports the preterrestrial nature of these ALH carbonates, which is also clear from their petrology (Mittlefehldt et al. 1994). We also combusted of a sample of ALH84001 which had been etched with phosphoric acid to remove any carbonates (Jull et al. 1998c). The carbon dioxide released by acid etching gave 337 ppm carbon as carbonate, with δ13C of +37.10 ± 0.01‰. This yield can be favorably compared to the 367 ppm carbon released in the previous stepped combustion of ALH84001 between 430 and 600 °C. So, this confirms that our temperature fractions give similar amounts of carbonate in 430– 600 °C. These results are shown in Figure 11. The results confirm the general trend already observed, that the organic material combusting between 75–400 °C is isotopically light, −31.9 to −25‰ and the fraction of modern carbon in these different steps is 23−40% modern (equivalent to 7400–11,900 yr old). Above 400 °C, our study indicates that the 13C-enriched carbonate is removed by acid etching, δ13C values of −14.7 and −8.1‰ and low 14C suggest the possible presence of another component. This small component appears to be an indigenous organic material combusting at higher temperature. This is likely some higher molecular weight material. Interestingly, Gilmour and Pillinger (1994) found similar δ13C values for polymeric material from the Murchison carbonaceous chondrite. This material would then be extraterrestrial, but not biogenic in origin. Radiocarbon Beyond this World 167

300 250 200 150

C, ppm 100 50 0 0 100 200 300 400 500 600 700 800 50 40 30 20

C 10 13

δ 0 -10 -20 -30 -40 0 100 200 300 400 500 600 700 800 1.0

0.5 Fraction modern C modern Fraction

0.0 0 100 200 300 400 500 600 700 800 Temperature, oC Figure 11 Stepped-combustion studies of an acid-etched residue of the Allan Hills 84001 Mars meteorite. Low-temperature steps indicate the release of terrestrial contaminants containing more 14C. The lowest 14C release occurs in the 400–500 °C step, which appears to indicate an extraterrestrial organic-like component. Adapted from Jull et al. (1998c).

McKay et al. (1996) only directly studied small portion (~1%) of the organics in the form of polycyclic aromatic hydrocarbons (PAH) in ALH 84001. The small size of this fraction precludes 14C measurements on these phases. In a separate paper published in the same issue of Science as our paper, Bada et al. (1998) studied amino acids in ALH84001. They concluded that the amino acids in this meteorite were all of the left-handed variety. All amino acids used biologically on Earth are of this type. This suggests contamination also, although amino acids from non-Earth life (if they exist) might be either left or right-handed. However, they could not be an equal mixture of right and left, which would indicate an origin from non-biological processes. 168 A J T Jull et al.

Becker et al. (1999) have recently reexamined the isotopic composition of the acid-insoluble component in ALH84001. They found that the acid-soluble component of the organics had δ13C of –26‰, whereas the acid-insoluble component was −15‰. The latter result was consistent with the δ13C values for the acid-resistant organic matter combusting at 400–500 °C reported by Jull et al. (1998c). Some differences in yield are apparent, Becker et al. (1999) used 1N HCl as opposed to the phosphoric acid used by Jull et al. (1998c). Recently, we have also studied bulk combustions of the observed fall, Nakhla. This meteorite fell in Egypt in 1911, and was collected in that year and 1913. Studies of the bulk combustion of a sample of this meteorite stored in the Natural History Museum indicated that ~80% of the organic material in this meteorite is of recent terrestrial origin. Contamination for the pre-bomb period 1911– 1950 AD ought to be ~98% modern. More recent studies (discussed by Jull et al. 1999b) on a freshly cut piece of the meteorite suggest that 14C could be used to demonstrate that much of the organic material was extraterrestrial, using the rationale discussed above. The organic material in Nakhla appears to be soluble in strong acids, and may have some similarities to the polymeric organic material found in carbonaceous chondrites. Obviously, more work on the isotopic composition and characterization of this material will need to be done in the future.

CONCLUSION 14C and other cosmogenic radionuclides have valuable applications in the extraterrestrial as well as the terrestrial realm. Cosmic-ray production of radionuclides in space gives us important information both about the history of cosmic rays, but also about the history of meteorites after they fall to the surface of the earth, as well as their infall history.

ACKNOWLEDGMENTS We are grateful to the National Aeronautics and Space Administration (NASA) for provision of lunar samples, and to the Meteorite Working Group (USA), Smithsonian Institution, National Insti- tute of Polar Research (Tokyo), Max-Planck-Institut für Chemie (Mainz), Western Australian Museum, and Natural History Museum (London) for provision of meteorite samples. We also acknowledge L J Toolin, A L Hatheway, L R Hewitt, T Lange, and D Reines for technical support of this work. This work was supported in part by grant NAG5-4832 from NASA and partly from grants EAR 95-08413 and EAR97-30699 from the National Science Foundation. We are also grateful to support for undergraduate research work by S E Klandrud, S Cloudt, and E Cielaszyk from the Arizona Space Grant College Consortium.

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WHAT FUTURE FOR RADIOCARBON?

E M Scott • D D Harkness Department of Statistics, University of Glasgow and Scottish Universities Environmental Research Centre

ABSTRACT. In this short article, we summarize some milestones in the 50-yr-long development of natural 14C measurement. In the light of this appraisal we presume to hazard some personal opinions and forecasts as to where best opportunities might lie for future gains from the continued investment in applied 14C science. The technique and the journal are one and the same in this regard.

INTRODUCTION In the half century that has elapsed since the first reported measurements of naturally occurring 14C the breadth of applied carbon isotope science has grown almost exponentially; reaching out from archaeology through oceanography, Quaternary science, geography and indeed to embrace most if not all of those disciplines that fall within the wide spectrum of “Earth science”. At the root of this opportunity for academic contribution is of course the ubiquitous role of elemental carbon in the very biogeochemistry of life itself. An early recognition of the importance of the radiocarbon dating method is perhaps best evidenced by the rapid expansion in the number of laboratories prepared to accept the considerable technical challenge posed by the need for the quantitative and reproducible detection of 14C in various natural materials. This via radiometric counting of weak energy beta emissions occurring at ultra-low specific activities. Nevertheless, over the first two decades or so the number of active “radiocarbon dating laboratories” increased from single figures to over 100 which were by then distributed worldwide. An ongoing expansion in scientific demand served to drive appropriate technological developments and/or innovative approaches to isotope analyses per se. For example, those pioneer laboratories initially dependent on the preparation and monitoring of solid carbon sources were soon retool- ing to benefit from the advantages afforded by the newer gas proportional or liquid scintillation counting techniques. A parallel demand for measurements on an ever more diverse range of naturally occurring materials and often in conjunction with smaller samples came from a varied congregation of “user scientists” who were keen to expound new hypotheses and to ask the inevitable searching questions. As a general rule, the “radiocarbon provider community” responded accordingly to this perceived need for interdisciplinary collaboration. It did so mainly by the development and delivery of significant technical improvements and/or new analytical procedures. A major breakthrough in the technological challenge came with the advent of 14C AMS (accelerator mass spectrometry). Direct measurement of the relative isotopic abundance of natural 14C was no longer constrained by radiometric counting i.e., the need to achieve an acceptable analytical precision at specific activities of less than 12 dpm/ gmC. Populations of 14C atoms could now be monitored without having to wait around for individuals to pop off. The immediate advantages of much smaller sample size and significantly less experimental lead-times has opened up a veritable Pandora’s box of scientific opportunity for the applied science. Albeit there is disappointment in some quarters that, as experience has shown, the expertise of high energy physics has not been able to extend the age range of the radiocarbon chronology or for that matter to improve on the dating precision achieved routinely by established radiometric counting labs. Nature itself retains ultimate control over these aspects of the applied science by virtue of setting the relative isotope abundance and radioactive half-life of 14C.

173 174 E M Scott, D D Harkness

Looking back over the half century of radiocarbon exploitation a salient feature is the success story penned by the constant interplay and mutual stimulation of user demand and provider response. However, this observation poses an obvious question over the continuing evolution of radiocarbon science viz.,

Are there still technical developments to come in natural 14C measurement which will seed and/or satisfy new areas of scientific enquiry or has the applied science reached its apex? Our response here must be an emphatic no! The road ahead for natural 14C measurement would seem to lead through many fertile fields ranging from those unfinished tasks in reconstructing the last 50,000 years or so of Earth history, through the ever more evident need for improved management of our planetary resources. We hope our attempt at a predicted route map, although entirely personal will nevertheless resonate with some of our readers.

Some Markers in the History of Applied 14C Science The scientific value of radiocarbon stems from the remarkable coincidence of several natural phe- nomena. First and foremost is the very fact that the fundamental element of life (carbon) occurs in nature with a radioactive isotope. Moreover, the characteristic half-life of this radioactive label is set conveniently within a range commensurate with the significant progress of human civilization. The geochemistry also appears as having been similarly contrived to set the challenge of radiocarbon dating viz., the continual cosmogenic production of 14C allied to its virtually immediate atmospheric homogeneity and direct biogenic availability in the gaseous phase. Of course the natural system is not ideal in all respects. We would all like to experience a greater 14C natural abundance than the somewhat parsimonious 10−12, and while archaeologists might be better served by a lesser half-life, nature’s determination of 5730 yr tends to be a wee bit short for geological preference. Archaeology provided the original driving force for development of the dating method since archaeologists were then in sore need of a secure time-scale on which to base and compare theories of succession and/or contemporaneity which were otherwise almost entirely dependent on assumed similarities among the physical debris of pre-history. Although archaeology remains prominent in the chronology’s user group, scientific interest in the radiocarbon clock has seen a gradual shift to the reconstruction of natural rather than cultural episodes and pertaining to the past 50,000 years of Earth history. This change in emphasis for the applied science reflects the growing tendency to har- monize and integrate man into the carbon cycle; his interaction with the environment forces changes, but in the past, natural environmental changes have also forced changes in customs, and caused populations to migrate and /or adopt new habits. Among typical user groups there has been a growing awareness of the fundamental importance of sampling integrity and in context of the true association between the measured 14C age of the selected material and the event that it assumed to date. The more ready availability of age measurements and the removal of size restrictions via 14C AMS has encouraged a move away from dependence on dating single samples pertaining to a recognized event. It is now common to encounter 1) the multiple dating of discrete physical or chemical components isolated from a given sample, and 2) the interpretation of series and /or sequences of related dates using complex mathematical methods. For the experimental chronologist there still exists the incentive to improve on both the analytical quality and user confidence in the objective interpretation of results. High on the agenda for the past 30 years or so has been the concerted effort by several specialist laboratories to chart accurately What Future for Radiocarbon? 175 those now well recognized wrinkles in the conventional 14C time-scale that derive from past changes in natural 14C production and/or transient perturbations in the dynamics of the natural carbon cycle. This high-precision calibration of the conventional radiocarbon time-scale against the absolute and continuous calendar provided by dendrochronology is painstaking work which continues even today (see the INTCAL 98). Furthermore, the need for extension of the 14C calibration curve beyond the availability of suitable tree-ring sequences has encouraged a focus on more recent research which seeks to exploit the independent timers inherent in the growth structure of corals and/or varved sediments and also explores the relationship to other naturally occurring radionuclides (a comprehensive account of progress in these areas will feature in a forthcoming issue of this journal). A concern for improved user confidence has stemmed from the continuing need to emphasize that natural radiocarbon measurement and in particular its interpretation as years past is not an absolute tool. It must be recognized that there is an inherent level of uncertainty in all results which depend on quantifying trace amounts of a radioactive isotope. It becomes incumbent on the measuring laboratory to ensure that its user scientists have an appropriate appreciation of both the accuracy and the precision that can be assigned for the reported conventional age and/or radiometric enrichment (e.g., pMC) values. In this context it is very satisfying to note that the vast majority of operational radiocarbon laboratories now address this essential aspect of customer care by their open participation in previous and ongoing intercalibration studies. Most recently in terms of the novel use of low-level 14C measurement is the exploitation of AMS technology in biomedical research. In particular, the quantitative tracing of processes that are essential to human metabolism e.g., nutritional effects, dietary studies and drug kinetics. While such work is to be commended and encouraged as being both intellectually exciting and socially satisfying it falls more in the category of the laboratory contained tracer study, perhaps then best regarded as a positive spin-off from the art of natural radiocarbon measurement.

Where are We Now? What is the state-of-the-community message after neigh on fifty years of technical development and practical experience in natural 14C measurement? In essence:

We can employ a universal chronometer covering the past 50,000 years of Earth history. However, as discussed previously, nature seems to have set this as the upper practical limit by virtue of half-life and natural abundance. The need for a total avoidance of trace modern carbon in both field sampling and laboratory based procedures (the inevitable background question) is a comparable analytical constraint and irrespec- tive of technical sophistication. We can achieve as a matter of routine an analytical precision of better than 0.5% and do so for samples that provide somewhat less than 1mg carbon. Thus ages pertaining to the last 12,000 years or so are reproduced with an analytical confidence of a few decades. Given sufficient sample size, the high-precision technology developed for dendrochronological calibration can achieve ±15 years or less where this is justified by sample integrity. Alternatively, under favorable conditions, a virtually absolute dating option is available via allied mathematical fixes such as wiggle matching to the mas- ter calibration curve. We can, as already mentioned, achieve a 14C measurement from even microgram amounts of carbon. However, it must be questioned whether there is a danger that, for the unwary user, this could represent a step too far in technological capability? Small may be beautiful and exciting but in many applied contexts there must surely be a limit below which the ultra-small carbon sample cannot be assumed to truly represent the particular event or natural process. 176 E M Scott, D D Harkness

Perhaps present day levels of attainment in the technical aspects of natural 14C measurement should be accepted as being at or close to our ultimate objective.

Where are We Likely to Go? The short answer is, “still a long way”. However, it seems to us inevitable that the intellectual challenge of natural 14C measurement will shift even more from technological improvement towards the realms of scientific application. There would seem to be more than enough opportunity for research satisfaction here and particularly so if the already obvious trends for amalgamation with other chronological methods and/or the exploitation of radiocarbon's counterpart cosmogenic radionuclides continue. A summary overview of some applied 14C research opportunities can be set most conveniently in context of the natural carbon cycle since this concept offers a convenient framework in which to sketch the complex bio-geochemical interplays that exist within and among the atmosphere, biosphere, and ocean. The atmosphere is of course the area of cosmogenic production and as such it will continue to be a primary focus for the study of those geophysical and extraterrestrial factors that have determined past variations in the production of radiocarbon and/or the other radionuclide byproducts of the primary cosmic flux. More immediate interest, and in relation to management of the global environment, centres on isotopic tracing and temporal quantification of the anthropogenic sources for those carbonaceous gases (CO2 and CH4) liable to contribute to climate change, and in determining the nature and ultimate response capacity of their natural sinks. Radiocarbon must stand ready to input unambiguous information and answers for future political debate centred on the “Kyoto Question”. Likewise in context of insidious pollution, is the need to monitor the significant and long-term radiation dose rate being delivered from 14C produced during the nuclear weapons testing programs together with localized emissions from our continuing dependence on the nuclear fuel cycle. Since there is a direct and virtually instantaneous coupling of atmospheric carbon to the tissue of living plants, the well mixed CO2 reservoir is effectively the primary source for all biologically recorded variations in 14C concentration whether this is a consequence of natural forces or human perturbation e.g., the “Suess” and “bomb” effects. The terrestrial biosphere is considered here as comprising all carbon contained in those chemical structures that can be ascribed to an organism that has lived at sometime during the range of the radiocarbon time-scale i.e., the past 50,000 years. It of course includes all plants and animals that are presently alive on land and in freshwater systems. This carbon reservoir affords by far the greatest range of opportunity for improving established research themes and the development of novel approaches. Two broad categories exist viz., 1) the ordering and/or reconstruction of past events, and 2) quantitative tracing of the modern pathways and/or fluxes of environmental carbon. In the first category we have of course radiocarbon dating support for archaeology where continuing emphasis must surely be towards improved confidence in the conversion of conventional ages to calendar dates. Confident calibration is essential to meet a changing culture within archaeological research in which understanding the relationships between cultures and cultural change, and delin- eating sequences of events or activities takes priority over simple dating of a proxy sample. What Future for Radiocarbon? 177

We have already touched on the incentive to extend the radiocarbon calibration curve beyond the limit of direct dendrochronological comparison. The obvious call here is for continuing research and development collaboration between 14C practitioners and their counterpart scientists with expertise in appropriate chronological comparators e.g., U/Th dating of corals, Ar/Ar dating, TL and/or OSL techniques and the recovery and counting of varve and ice-core sequences. There is also a reverse angled viewpoint concerning the importance of the calibration data set. Past temporal changes in natural 14C concentration provide a unique index for comparison of the global synchroneity, rates of change and possible driving mechanisms that obtain for those episodes of major climate change that are variously recorded in many sedimentary archives. A prime example here concerns interpretation of the complex patterns associated with the last transition from full glacial to temperate conditions around the north Atlantic margins (see Lowe and Walker in this issue). Such exercises in environmental reconstruction call increasingly for the development and application of appropriate expertise in the identification and recovery of the most representative components within the bulk organic matrix and in relation to a diversity of host deposits e.g., lake sediments, peat accumulations, paleosols, loess deposits, ice-cores, etc. A particularly challenging and potentially rewarding approach would seem to be with prior compound specific isolation and geochemical definition of the measured organic materials. Anthropogenic disturbance of the near steady-state distribution of natural radiocarbon viz., via 14 emissions of fossil fuel derived CO2 (Suess effect), injection of the “bomb C” spike and massive deforestation programs, still affords unique but transient opportunities to trace carbon transfer rates with an otherwise impossible degree of temporal resolution. The immediate exploitation of the “bomb pulse” to quantify atmospheric mixing patterns, air-to-ocean exchange parameters, and the dynamics of oceanic circulation are well documented. However, the continuing distribution of this excess of artificial 14C through vegetation and soils is still useful for evaluation of the associated organic carbon dynamics. This via the construction and testing of models of carbon flux that can be applied to ensure best practice in the future management of agricultural and forest ecosystems. Although not all of the carbon contained in groundwater can be ascribed to a biological source it is appropriate to feature the importance of natural 14C measurement in the management of aquifer resources in context of terrestrial deposits. This highly specialized field presents a considerable but entirely satisfying challenge for isotope geochemistry (see review by Geyh in this issue). There can be no doubt that the social and economic importance of adequate water supply is sufficient stimulation for continued research and development effort geared to improve management options. The World’s oceans contain and cover the largest reservoir by far of chemically active carbon. This inventory has been the subject of intensive and productive 14C based research over several decades. Included here has been the charting of circulation patterns, the determination of mixing rates and the dynamics of deep-water formation which was in turn aided by the net input and subsequent distribution of “bomb 14C” tracer. An important outcome of that work has been an appreciation of the paramount role of oceanic mixing patterns and their associated heat transfer capacities in determining the timing and extent of past climate change. Such evidence provides strong justification for continued research focused on the complex carbon isotope geochemistry, both inorganic and organic pools (DIC/DOC/PIC/POC), that characterize discrete water masses. Likewise, the history of past changes in ocean status within the regional environment is retained in the stratified seabed sediments. 178 E M Scott, D D Harkness

CONCLUSION In their foreword to the volume published to mark 40 years of radiocarbon science1 editors R E Tay- lor, Austin Long, and Renee Kra (1992) highlighted a citation made in support of Willard Libby’s nomination as Nobel laureate viz.,

Seldom has a single discovery in chemistry had such an impact on the thinking of so many fields of human endeavour. Seldom has a single discovery generated such wide public interest. After a further decade of extremely interesting and productive developments in applied 14C measurement the sentiment is equally if not even more appropriate today. The international “radiocarbon family” continues to grow and prosper. At the firm foundation of this success is the scientific kinship forged through interdisciplinary stimulation and allied to an attitude of unstinting collaboration. The necessary home for this success has been provided by the journal Radiocarbon and in practice via its marriage of sound editorial policy with the highest levels of professional publishing practice. Here’s to the next 50 years!

REFERENCE Taylor RE, Long A, Kra RS, editors. 1992. Radiocarbon after four decades: an interdisciplinary perspective. New York: Springer-Verlag Inc. RADIOCARBON UPDATES

New Laboratory A new laboratory has opened in Miami, Florida. Ronald Hatfield and Darden Hood are the Directors of BIOCAMS International, which provides “AMS dating designed exclusively for art, antiques, and materials of potential commercial value”. See http:/www.biocams.com/ for details.

New Accelerator Mass Spectrometer In March 2000, the University of Arizona NSF-AMS laboratory began running preliminary dates with its second accelerator unit, a 3MV Pelletron, manufactured by NEC.

Australasian Conference The next Australasian Archaeometry conference will be 5–9 February 2001, at University of Auck- land, in Auckland, New Zealand. The conferences are held every four years, and this will be the first time the conference is held outside Australia. In 1997 this conference was attended by several hundred scholars with involvement in the fields of Archaeology, Anthropology, Geography, Conserva- tion, Museology, Material Science and Applied Nuclear Science (e.g. dating, materials analysis etc).

LSC 2001 Conference The next liquid scintillation conference will be held 7–11 May 2001 in Karlsruhe, Germany. Prelim- inary registration begins in June 2000. Papers are due to the conference secretariat no later than 1 October 2000. Please see the announcement towards the back of this issue.

Saskatchewan Lab Closure After 45 years, the Saskatchewan Research Council Radiocarbon Dating laboratory (lab code S) has ceased operation. Below is an overview of the lab’s history by Richard E Morlan of the Canadian Museum of Civilization.

The Saskatchewan Radiocarbon Laboratory was the first radiocarbon dating facility in Canada, commencing operation in the early 1950s at the University of Saskatchewan, Saskatoon, under the direction of K J McCallum. Initially, the Libby technique of counting solid carbon samples in a screen-wall counter led to considerable difficulty with sample preparation and contamination from radioactive fall-out, and the laboratory soon opted to install gas proportional counting equipment (McCallum 1955). The routine operation of the new equipment was taken over by Jurgen Wittenberg who continued in this role for the next three and a half decades. From the outset, the Saskatchewan Laboratory was supported by the Saskatchewan Research Council (SRC) and, occasionally, by the National Research Council of Canada. In the early 1970s, the Laboratory began to operate commercially under the administration of the SRC and the direction of A A Rutherford. Soon thereafter, in order to provide radiocarbon dating to Canadian archaeologists, the National Museums of Canada contracted with the Saskatchewan Laboratory for their services. By the end of the 1980s, Jurgen Wittenberg retired and the National Museums corporation was dismantled, thus direct support to the Lab- oratory was withdrawn. These two events were not directly related to one another, but it was no mere coincidence that the gas proportional laboratory discontinued operation. In 1989, the Laboratory was re-incarnated off-campus in the new facilities of the SRC. At the new location, a liquid scintillation counter was employed under the direction of Gene Smithson and the technical expertise of Jeff Zimmer. This change in counter technology took place shortly after S-3000 was pro- cessed. Some age determinations were verified by both gas proportional and liquid scintillation counting

179 180 Radiocarbon Updates

during the transition period, but samples after S-3051 have been dated by liquid scintillation (J Zimmer personal communication 1998). The last sample number assigned by the Laboratory was S-3669. Most of the work of this Laboratory has been devoted to Canadian Quaternary research and especially to archaeology and vertebrate paleontology. Even with many of its recently analyzed samples not yet available in the scientific literature, 64% of the total number of dates (3669) have already been entered into the Canadian Archaeological Radiocarbon Database (http://www.canadianarchaeology.com/radiocarbon). Of more than 7300 dates in CARD, 2142 (29%) were dated by the Saskatchewan Laboratory. Of the latter, 1094 (51%) are samples of bone, tusk, horn, or antler. Rutherford and Wittenberg (1979) undertook extensive studies of bone pre-treatment protocols. Begin- ning with S-1300, collagen extractions were solubilized using a method similar to that of Longin (1971). It is noteworthy that studies relying heavily on bone dates from the Saskatchewan Laboratory (e.g., Mor- lan 1993; Dyke et al. 1996) have established very coherent chronologies for the histories of northern Plains cultures and Arctic sea mammals. While celebrating the enormous contribution of the Saskatchewan Laboratory to Canadian Quaternary studies, we must reflect that the decommissioning of this Laboratory represents an enormous loss, especially in view of so many previous closures of radiocarbon laboratories in Canada. It’s getting hard to get a Canadian date! Richard E Morlan Canadian Museum of Civilization PO Box 3100, Stn B Hull, Quebec J8X 4H2, Canada

Selected References Dyke AS, Hooper J, Savelle JM. 1996. A history of diocarbon dates in Saskatchewan. Saskatchewan sea ice in the Canadian Arctic Archipelago based Archaeology 13:2–84. on the postglacial remains of the bowhead whale Rutherford AA, Wittenberg J. 1979. Evidence in sup- (Balaena mysticetus). Arctic 49:235–55. port of soluble collagen extraction for radiocarbon Longin R. 1971. New method of collagen extraction bone dating. Saskatoon. Saskatchewan Research for radiocarbon dating. Nature 230:241–2. Council Report C79-22. 8 p. Morlan RE. 1993. A compilation and evaluation of ra-

Saskatchewan Date Lists McCallum KJ. 1955. Carbon-14 age determinations at Rutherford AA, Wittenberg J, McCallum KJ. 1973. the University of Saskatchewan. Transactions of University of Saskatchewan radiocarbon dates VI. the Royal Society of Canada 49 (Series III, Section Radiocarbon 15(1):193–211. 4):31–5. Rutherford AA, Wittenberg J, McCallum KJ. 1975. McCallum KJ, Dyck W 1960. University of University of Saskatchewan radiocarbon dates VI Saskatchewan radiocarbon dates II. Radiocarbon [sic]. Radiocarbon 17(3):328–53. 2:73–81. Rutherford AA, Wittenberg J, Wilmeth R. 1979. Uni- McCallum KJ, Wittenberg J. 1962. University of versity of Saskatchewan radiocarbon dates VIII. Saskatchewan radiocarbon dates III. Radiocarbon Radiocarbon 21(1):48–94. 4:71–80. Rutherford AA, Wittenberg J, Wilmeth R. 1981. Uni- McCallum KJ, Wittenberg J. 1965. University of versity of Saskatchewan radiocarbon dates IX. Ra- Saskatchewan radiocarbon dates IV. Radiocarbon diocarbon 23(1):94–135. 7:229–35. Rutherford AA, Wittenberg J, Gordon BC. 1984. Uni- McCallum, KJ, Wittenberg, J. 1968. University of versity of Saskatchewan radiocarbon dates X. Ra- Saskatchewan radiocarbon dates V. Radiocarbon diocarbon 26(2):241–92. 10(2): 365-378. LSC 2001

International Conference on Advances in Liquid Scintillation Spectrometry

7–11 May 2001, Karlsruhe, Germany

LSC 2001 continues the series of conferences most recently held in Gatlinburg, Tennessee, USA in 1989, Vienna, Austria in 1992, and Glasgow, Scotland in 1994. Liquid scintillation, with its recent developments in low-level counting and α/β-discrimination, has become a key technique in radiation counting and β-spectrometry. This conference will provide a forum for radioanalysts to discuss their most recent findings and future work, either as oral lectures including both invited and contributed papers, or as posters. Poster authors will have the opportunity to give a short presentation before each poster session. The conference will be held in English. The conference site will be Karlsruhe, Germany, a beautiful historical city at the northern edge of the legendary Black Forest, near the city of Heidelberg with its famous castle and the lovely Neckar valley. Sightseeing tours will be offered to participants and accompanying persons. The conference fee will be approximately US$ 400 for active participants. This includes a welcome reception, coffee during session breaks, excursion fees (Heidelberg, boat tour on the Neckar river), the conference dinner, and snacks during poster sessions. A reduction for students is foreseen. To receive the next circular, please register on our website.

Call for Papers One-page (A4) abstracts (including title, authors and their affiliations, text) should be forwarded to the conference secretariat, preferably via e-mail, before 1 October 2000. Authors should note that manuscripts must contain original data and should emphasize novel aspects (rather than routine application results). See our website for more information and authors’ instructions.

Conference Topics Conference Website • New Instrumentation http://www.ftu.fzk.de/lsc2001/ • Environmental Applications and Analysis • Bioscience Applications and Medicine Conference Secretariat • Health Physics Applications • Natural Radioactivity with Special Siegurd Möbius Emphasis on Radon Forschungszentrum Karlsruhe GmbH • Alpha Counting FTU • Cerenkov and Luminescence Counting Postfach 3640 • Sample Preparation and Cocktails D-76021 Karlsruhe, Germany • Other Scintillation Techniques (The list of topics is certainly not exhaustive Tel: +49 7247 823791 and is for informative purpose only.) Fax: +49 7247 824857 E-mail: [email protected]

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17th International Radiocarbon Conference

It is our great pleasure to invite you to participate in the 17th International Radiocarbon Confer- ence, scheduled for June 18–23, 2000, in Israel. The Conference will be held at a beautiful location, in the rural setting of Kibbutz Ma’ale Hachamisha in the Judean Hills, just west of Jerusalem. The Kibbutz offers an attractive self- contained arrangement of excellent accommodation and conference facilities, which will enable a high degree of interaction between conference participants. The City of Jerusalem with its unique history and tourist attractions is a short drive away and is easily reached by bus or taxi. The first 14C Conference at the dawn of a new millennium will undoubtedly include exciting new scientific developments. Keeping the tradition of past Radiocarbon conferences, the scientific program will include a wide variety of topics. Sessions will be devoted to: • Archaeology – with a special session on 14C data of • Global change historical periods in the Near East •Glaciology • Calibration of the 14C time scale • Hydrology • Sample treatment and measurement techniques • Oceanography • Geophysics and Geochemistry of 14C •Geology • Cosmogenic radionuclides •Soils • Environment past and present Participants are welcome to indicate their preference for either oral or poster presentation of their papers. However, the final decision regarding form of presentation will be made by the Organizing Committee. Detailed information will be included in the Second Announcement. The social program of the Conference will include an afternoon walking tour in the Old City of Jerusalem and a one-day tour in the unique Dead Sea area (lowest point on the earth’s surface). A floating swim in the Dead Sea – which is seven times saltier than the ocean – is indeed a unique fun experience. Sincerely, The Organizing Committee: Israel Carmi, Chairperson, The Weizmann Institute of Science Elisabetta Boaretto, Secretary, The Weizmann Institute of Science Hendrik J. Bruins, Ben Gurion University of the Negev Michael Paul, Hebrew University of Jerusalem Dror Segal, Hebrew Antiquities Authority Yoseph Yechieli, Geological Survey of Israel For further information, please see the Conference website: http://www.radiocarbon.co.il/ 0KX[YGRKS .*-:&9* RADIOCARBON $5 Back Issues Clearance Sale We found hundreds of copies in an old shed! Priced to move.

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